And with respect to the incest, L&P has already shown that "between us girls", it's not seen as perhaps as taboo as most might consider it. Since there's no consanguinity, the normal taboos aren't quite as applicable. Like with adultery, since by it's original definition only applies if the wife sleeps with another man (still cheating though).
Yeah, I'd totally agree that the point you're making here is the only somewhat reasonable way to explain how Sophia could have intercourse with her sister, I just don't see Sophia going there on PD15 considering her current state of mind, internal monologues and overall progression in the story with her sister or anyone for that matter, really. I guess the big problem is that L&P gave himself only 30 ingame days, making it pretty much impossible to turn this into a believable story. I guess at some point he has to just fast forward things, believable or not.
Regarding your question as to what I'd consider to be lesbian sex, well, for hetero scenes that's pretty obvious. On the lesbian route, I'm thinking of penetration with fingers, oral, scissoring...Yeah, I'd say that'd be considered lesbian sex. I don't expect Patricia to whip out a strap-on dildo for Sophia's first incestuous lesbian sex scene. Then again...It's Patricia we're talking about.
I'm not entirely ruling lesbian sex with Sophia on PD15 out, I just don't believe that L&P will make it happen as it would require alot of progress on the corruption front within the next 1,5 PD's, imo. But yeah, I guess the "between us girls" argument could totally work out. Atleast I'm hoping for your sake that it does. I myself am not that excited about the lesbian scenes anyway.
Edit: Oh, and not to forget that I strongly believe in L&P's plans to postpone sex scenes with Sophia for as long as possible for various reasons, but you know my stance on that topic.