1. Do you give it much importance?
The Sun/Moon midpoint is the
most important midpoint.
It represents the balance between the soul-nature (Moon) and ego-nature (Sun). All the other meanings of these two objects apply.. The balance between father and mother roles, make and female, conscious and subconscious, will and emotion, radiant vs reflective. I'm sure you're familiar with how the Sun and Moon compliment eachother in a number of ways, so think of the midpoint as their coming together.
The Sun and the Moon are natural partners, as they compliment eachother. Whenever two natural partners pair up as a midpoint, you generally have a very important spot in the chart. The same can be said of Venus/Mars (the sexual attraction midpoint), Jupiter/Saturn (optimism vs pessimism) Uranus/Neptune (individual vs communal) and a number of others pairs as well.
An example would be found in my progressed chart, where Venus is presently right on my Sun/Moon midpoint, which has invoked an ongoing balancing and harmonization of my masculine vs feminine nature.. A wonderful experience.
But the Sun/Moon midpoint is so sensitive and all-encompassing (it could be said that the combination of Sun/Moon energy define a large percentage of an individual, as they are the
luminaries afterall!) that a planet at this midpoint will express itself throughout the native's experience, either by natal placement, transit or progressions.
Ebertin lists SO/MO as "Spirit and soul, the conscious and the unconscious, vitality and feeling, husband and wife"
When positive, a harmonious character, inner balance
When negative, inner discontent, an unbalanced nature, the struggle between duty and interests, tendency to allow inner conflicts to manifest. He goes on to state that the sign of the connection has a large influence of it's manifestation, and lists sociological correspondences as "husband and wife, father and mother, boyfriend and girlfriend, marriage, the parent, friendship"
2. Does the interpretation of a planet conjunct the Sun/Moon midpoint in two charts differ if the relationship in question is a sibling to sibling, instead of a romantic relationship?
Yes, of course, as the interaction of the natives in question would be very different. Venus in this sort of synastry among a couple may result in a strong love bond as a manner of bringing harmony, but could very well indicate a strong cooperative musical connection between friends, as both interpersonal love and music are expressions of Venusian harmony.
The Venusian principle is harmony, so she will find a way to harmonize the other's Sun/Moon principle, but of course the method depends on the sort of social interaction between the two individuals. Planetary energies always play out through the channels of human interaction between people, and one must remember that each object in the chart rules a wide varieties of objects, activities, emotions and colors of personality reality. A little synthesis will go a long way
3. What is the interpretation of the North Node or Chiron in one person's chart, conjunct the Sun/Moon midpoint in the other person's chart?
I try to avoid using receptive or calculated points (the angles, nodes, vertex, etc etc) to keep things a bit cleaner and because I'm not sure exactly how important they are in midpoints. My visual conception of a midpoint requires two energy generators, but others may disagree.
I would, however, consider Chiron would have a strong meaning at this point. The Chiron person would likely play an active role to both shed light on the other's Sun/Moon imbalances, as well as a catalyst to foment the healing of those imbalances. As usual for Chiron, that process may take place as a series of deeply personal and potentially troubling experiences which lead the native in a soul-level manner to recognize the challenges he faces to overcome his deepest fears or shortcomings. The Chironic journey is seldom apparent at the time and is often only understood by hindsight, as it is soul-level and subtle; even if the experiences are extremely obvious, the dots aren't always connected until later. The meaning may not be understood until after the healing process has taken hold, and the native is able to consciously engage the healing path, and often not until the healing path has mostly been completed. Many of us don't really understand our Chironic journey until close to the Chiron return, around age 50. I'm presently in the upper square of the journey and still have barely figured it out, although the path is fairly apparent, the endgame is still not.
It could also indicate the Sun/Moon person will help to further the Chironic journey of the Chiron person, applying the power of their luminaries to the other's Chiron.
It really depends on what's going on both between the individual and each individuals path and transits, progressions, etc. One should not try to enact a canned interpretation without a complete analysis of the individuals in question, especially with Chiron, aka the "maverick"! A little analytical creativity is advised, as is the use of your intuition as to where the two folks are at. Say, for example, that the Chiron individual meets the Sun/Moon individual when he is undergoing a powerful Chiron/Chiron transit (say Chiron-square-Chiron), or an outer planet is conjuncting the natal Chiron, it would be obvious the Sun/Moon person has entered at a critical Chironic phase for the Chiron native, and thus is going to further that journey, or atleast be an important part of it. However, if one person has transiting Uranus at his own Sun/Moon midpoint, thus conjuncting the other's Chiron, it would then be obvious the Chiron native is part of the UR=SO/MO cycle of the native undergoing the Uranus transit.
In short, take all the cycles into consideration to see how the midpoint synastry may play out. First meeting charts and charts for other important points in the relationship can be most revealing, as can the composite and davidson charts.
4. What about the Sun or Moon in one chart, conjunct the Sun/Moon midpoint in the other chart?
Simply put, a classical marriage indicator or very strong bond between two individuals!
5. Does the Sun/Moon midpoint aspecting a planet between two people's charts feel different to each person?
For example, does the relationship seem karmic/significant to the person whose midpoint is aspected, whereas the person who's planet is doing the aspecting views the relationship in a less serious way?
It really can work either way. The energy may have a stonger flow from the two planets into the singular planet, or from the singular planet to the midpoint pair. One must look at those objects in question to determine their strengths. If the singular object is immensely powerful it may tend to direct the midpoint pair, but if the midpoint pair is strong and is natally activated by a third planet (say if Mars squared the Sun/Moon midpoint in a single native's chart) it could very well play the lead role over the other's planet. I've seen this work both ways, and again one really must be creative and holistic in their interpretations of midpoints, and the interaction is always atleast somewhat bidirectional, if not balanced.
How serious it seems to either party would largely depend on how powerful and important the objects in question are. If the planet is weak in the chart and not expressed fluidly by the native, the midpoint aspect might not even play out in a notable manner, but if the planet is the native's main focus, the singular midpoint aspect may be the key focus on the relationship.
Anyways, I'm not a midpoint expert but have been working with them for a little while, yet still much to learn. I generally look to the more apparent placements, such as regular transits and synastry aspects, but sometimes find nothing apparent. I then turn to midpoints, and more often than not I find a very obvious, tight orbed midpoint configuration which just screams of the situation.
A good example would be an acquaintance who recently lost her grandfather, who was very important to her. It wasn't very apparent, but when I found Chiron within a few arcmin of her Sun in Capricorn (symbolic of an old man/father figure) and Venus in Pisces (a dearly loved one of all-encompassing depths) when this event happened, it became obvious his loss was part of her Chironic journey and the situation was very much playing out as such. This lined up with her first Uranus-Uranus square, showing that his passing is going to help her build her individuality, which seems obvious now that she's very much "on her own" in a personal emotional sense.
Just be careful not to get too deep into midpoints as to muddy the picture; avoid the calculated points and angles, and only consider 0, 90 and 180 degree aspects to them, with a maximum orb of a single degree, focusing on midpoints within a half degree or less. Honestly, unless I'm trying to understand a specific circumstance I try to limit them to conjunctions and oppositions within a 1/2 degree orb, aside from the Sun/Moon midpoint where I consider all 5 8th harmonic aspects (0/45/90/135/180) at up to 1 degree.
I hope that helps, and that I didn't further confuse you, and that all this made sense as I'm still waking up a bit today