I'm doing the game again right now, I left the game when the sister left the town(near version 11 maybe) ... So you are saying that if we let the relationship with thadeus (i don't really remember how far it goes, and i hate NTR xD) and we find all of her secrets, we might keep her after the magic scene ?1. Seems like it. So far as there isn't any consequences with sleeping around yet.
2. If your not into having a love interest being with someone else, there is one instance (so far) but you are given a choice of whether or not to let it proceed. There is a (same) character that if you take things too far before you learn all her secrets, it seems they are gone from the game permanently.
edit: just in case, I don't want to offend anyone here, I like the game ^^