her friendship and your debauchery, or maybe incest need to be high enough. NO idea how high at the moment, testing right now.
Edit:
- Marina needs a Friendship of 30 and a debauchery of ~40 (tested at 30, and 40 using cheats), and incest of 0.
- Inna also requires a Friendship of 30 and a debauchery of ~40, and incest 0.
- Vika also required Friendship of 30 and debauchery of ~40, and incest 0.
At this point I'm assuming friendship of 30, and debauchery of 40 for all characters. Incest is likely a gating mechanic for something else.