As someone already quoted the incest tag, the NTR rules for the tag read as,
Ntr [Designed to cause jealousy by having the romantic interest involved with someone other than the MC.]
I think the 3 sisters are clearly romantic interests at this point in the story, and the only reason the BF of Lilith exists right now is to make you feel jealous about it. MC has said several times now that he doesn't trust him and thinks he isn't a good person.
I think following the rules of the site, this game DOES deserve an NTR tag right now.