I'll endeavour to answer your questions as best I can.
Firstly, how to calculate the midpoint between the two sextiled planets ?? Unless you have a midpoint facility on your astro program, then it can be done in your head!! I calculated the MP of your Mars/Pluto to be 2 deg 45' in Virgo, so any transits to that point would be significant.
As for the name, Yod, I first read about it in a book I have - "A Spiritual Approach to Astrology", by Myrna Lofthus. Firstly, the name is derived from Hebrew. She says, and I quote:
"Esoterically, the name Yod means to experience a union with one's higher self. Because of this, it is apparent that those with a Yod, or Yods, in their chart have experienced this union in other lifetimes."
and further:
"Since "Yod" individuals have experienced their higher self in former lifetimes, the presence of a Yod indicates a desire of the soul to seek and perfect this union this lifetime."
And she goes on to say that the tarot card, The Hermit closely represents the Yod, in that
although the Hermit appears to be alone, he is really a leader with a gift for showing the way. I find that sentence to be quite profound. The key to this, apparently, is to find one's higher self through accessing the `positive' vibrations of each of the three planets involved. Now ... that sounds like a fine task, but could take someone all their life to achieve.
I think the advice from Myrna Lofthus about becoming familiar with your Yod planets is really relevant, because until you are faced with a struggle of some kind - often something which
feels like a life-or-death struggle - you will not be fully aware of how you are able to fully access that Mars and Pluto sextile. I hark back to Lindy Chamberlain, whose Yod experience was not only protracted, but deeply tragic, and the strength she showed once she'd been released from Darwin prison had her appearing like a different person; that was her accessing that Pluto-Mars-Saturn in Leo. She managed her ordeal with dignity, with no knowledge of astrology, which most people manage to do, chiefly because
no-one born with a Yod on their chart is unable to use it effectively when the time comes. Not only did she lose her newborn baby in tragic circumstances, but her 2 young sons were continually taunted at school, and eventually she and her husband divorced. That is extreme. Most Yod people don't have experiences like that. What is required during a Yod experience is honesty and patience.
I hadn't known that the journalist who was executed by ISIS had 3 Yods. That is remarkable. What a sacrifice. It would be interesting to do a search on people who have Yods. I could spend weeks doing that!
The question which seems to recur for you is how to work with the NN and the Yod, and how to spot the difference. My best advice is to not focus on them. It will drive you crazy, as you wait for the next `test'. The best preparation is to become knowledgeable about the three planets in your Yod, and how best to access them in a way which strengthens you. I used to obsess about my NN, but now I never think of it. I realise that I have moved towards it naturally. There will be all those others in your generation moving towards their own NN, so similar values are held by those of the same age. The drive of your soul is very strong.
As for discovering when the Yod planets will be activated by transits, I use the American Ephemeris, although I note that astro.com now has an ephemeris facility on the chart drawing page.
And now to your own early life experience with the multiple tragedy in your family. Grief of that magnitude is such a heavy atmosphere in which to grow up. It is unlikely that being positive would have sent it away, and I suspect that the grief still lurks. I also suspect that your sister's teenage behaviour was her own unconscious response to that tragedy.
[Just as an aside, my oldest son was labelled ADHD during his early adolescence. He's the one with Pluto-Moon opposition. He is also Cancer Sun with Pisces Moon, so he is highly sensitive to his environment. I had a massive struggle with him. Then one of my friends told me that she'd heard some psychologist say that kids with that kind of behaviour are mostly responding to the suppression of grief in their parent(s). I found that interesting, and probably spot on.]
I can fully understand your mother's choice to not surround you with sadness, but the fallout of so much loss always remains in some form. I am wondering whether there were other tragedies earlier in the family - say, several generations earlier - which were handled in a different way. If there is no way of knowing that then it doesn't matter. Your being nurtured through that time will have had a powerful impact on both you and your sister. I find the "raging tempers" interesting. That's Mars-Pluto, and it is probably a good thing you and your sister possess that. It provides strength of purpose (and more than a little outrage when required
) And no matter what you have to face in your life, nothing will be quite as tragic as the deaths of your father and grandfathers. I am also wondering about your mother's chart, especially since she lost her husband and father in close succession. Was she born with Chiron in Scorpio, as I was?
And you asked about my relationship with my oldest brother. We never got on. In fact, my other 2 older brothers didn't get on with him either. He was odd, probably mildly autistic (mild levels of autism is prevalent in the males in my family). We are/were both Leos, and there can never be more that one Leo in any family!! But his death became a tragedy in that earlier family tragedies were brought into the present, but not in a conscious way. I'm not even sure I can explain that, but that is how Yods work. We have a `memory' of what we have to do, even if we don't have conscious access to that memory.
I guess what I'm saying is that working with your Yod is not always going to be conscious, and when you are tested in any way, the strength you have gained from that Mars-Pluto sextile will provide focus, even clarity.
And I just noted you have natal Chiron in Aries in the 12th house. The level of sensitivity granted by having Chiron in the 12th will be a valuable tool for you.
And I hope I have answered your questions adequately. I am aware that being a Gemini you probably require a step-by-step guide through this, and there is no such tool available. I can only repeat what I said earlier ... If and when your life is held up by circumstances beyond your control, there are two things only you need to keep in mind; the first is to never compromise your moral code, and the second is to be patient, as the situation will not last forever.
And as conscious as I am about how loooong this post has become, I thought I'd add that I do have a Yod on my natal chart, but have tended to dismiss it because asto.com does not - and many others do not - recognise a Yod formed with Asc, MC or Chiron, however, Karen Hamaker-Zondag does, and so that's good enough for me. The apex of my Yod is my Capricorn Ascendant. It was in writing what I said about about The Hermit card that it became clear. What I wrote about "being alone, but being a leader with a gift for showing others the way" pretty much describes my life!! Perhaps that is also your role, Witchyone.