Hello oleanna, sorry it took me so long to reply. With your permission I am posting my reply based on the questions you pm'd me.
The first question you asked me was about the size of the houses, and whether or not I thought you should use whole signs for horary charts.
My answer for this is that you do consistantly get distorted houses at your location using regio, and probably any quadrant based house system. Personally, after reading the works of many ancient astrologers, I use both. That is to say, there was a distinct difference when Saul and Ezra and Masha'Allah were talking about places vs. houses. The places are the signs topically assigned as houses. The houses as we know them today are divided by cusps, and those change (aside from the ASC and MC, which are the same no matter what) depending on what flavor house system you prefer. My biggest beef with that is the chart is the chart is the chart, and significators shouldn't be different in different house systems.
That said, counting signs, or places, to find significators makes the most sense to me. As I have found this method to work, I'm not likely to change it.
Your next question was based on my reply to your pm, in that I suggested that you needed to look at the ruler of the 5th, since this chart was specifically about sex, and sex as a pleasurable act is the provence of the 5th house. You also expressed concern that because your sister/querent has a past history with this person you were concerned that there was some subtext in the question.
I understand that you felt a wee bit subjective with this chart based on what you know of the querent's history with this person, but that can be both a good and a bad thing. Good because you don't have to ask a million background questions to get the whole picture, bad because your personal opinion of the quesited/situation can color your reading. Your sister asked a bald faced question: Will we have sex. You understood as a sister that there could be subtext in the question, but as the astrologer did you ask for clarification? I only say this because it does help to know what the querent really wants from the quesited before you give her the answer. Is she really looking for it to be something more, or is she looking for a terrific romp in the hay?
So, let's look at the chart. The querent/your sister gets Saturn, in his second station in the 11th house. The guy gets Moon, emplaced in the 1st house (emplacement is when the quesited is in the ASC. For example, if you are asking about a job and the planet ruling the job is in the ascendant, then usually no matter what the rest of the chart says the answer is yes. In lost object horaries, if the missing object's significator is in the ASC, especially retrograde, then the answer is you will find it without trying. ) But in this case, the thing sought based on the question isn't the Moon, but rather Venus, because the question is clearly and specifically about sex. Venus rules the 5th sign, and therefor our chart is this
sister=Saturn, rx but in second station
guy=Moon, debilitated in Capricorn, applying to sextile rx Saturn who is in Moon's descension, albeit to a sextile.
thing sought=sex=Venus, just in her 1st station, wildly out of sect, separating from a square to rx Saturn who is in her debility after which she moves into his. Venus applying to conjunction with Jupiter, who rules the 12th of self undoing and is the exaltation ruler of the 7th.
You never really want to see the Moon applying to a planet in her fall, and when that planet is Saturn rx...just ick. Especially when the Moon is in Capricorn, her detriment.
It's really looking like it's in your sister's hands. As in, the significator of the person she's asking about is in the ASC, applying to a sextile with shakey reception to her, and if we check Bonatti's table it's the easiest manner of perfection out there. But you are right in your concerns, in that this relationship is a non-starter. There will be no anything else afterwards. So it is really in your sister's best interest to walk into something with eyes wide open. If she wants to have no strings attached sex with this guy then she can. But the odds are it will cause her more harm than a moment's pleasure is worth. You can caution her that it's not in her best interests to do this, but then again, since when do sister's listen to each other?
Was there a history of violence in this relationship? You asked via pm about the size of the 7th house in regio, and most of those 7th house planets, including the 5th sign ruler, are in the 8th sign, 7th house. This is a bit disturbing.