Ur dad (stepdad) will suicide if u get rid his wife (ur mom) u can choose if u let him die or help him (but he will go forever) and for Davide u need to convince one of ur sisters for tell ur mom that Davide rape one of them, then ur mom in one of the orgy event will attack him (careful because if u kill him u cant go more for the orgy event in the basement)
Obtain too much relationship points with ur mom and sisters (good relationships, because if u have bad points in this the events will change in NTR and u cant kill Davide)
For Frank (tans owner) I dont know how to kill or do anything against him.. I think atm we cant do anything.. but he has some NTR scenes with ur big sis, ur mom and ur friends big sis. Use the cheats if u dont want grind too much the points for relationships.
Ahm, one more thing, although u kill Davide, if u have bad relationship points with ur big sis and mother, if u decide go with them to sell drugs in the basement, Davide will appear and fucks them (NTR scene) so be careful