If I remember right, Sveta's sister is really easy though a bit of a grind to get without cheats. Pretty sure all you've got to do is buy dresses from the mall and gift them to her repeatedly until she decides you're better off with her instead of her sister...I think. And just in case people are dumb like I was, she lives in Sveta's original apartment after Sveta moves into the one you rent for her during her pregnancy.
And which neighbour do you mean? If you mean Shuriks mom, you have to exhaust all of the dialogue about Mothers with Shurik until you can spy on her with her husband. Do that a couple times and you'll complain there's no way to get her with her husband around. Then go to the park a couple times in early morning until a gypsy lady turns up. If you give her 3000, when you next go to Shurik's and try to enter the house, you'll find out the husband has to go away for work. Standard thing of increasing the Mothers depravity, staring at her tits until she let's you feel her up and then you have to go to the nightclub. You'll see her dancing there, choose to take her home and then say you do expect a reward for looking after her. Next time you visit her in the evening you can tell her you want to continue fucking her.
If it's Anastasia, you first have to visit them often enough to get a standing invitation to her home. Then on the weekends you can attend a poker game with her husband. Keep on winning and he'll up the stakes, until eventually the stakes are a night with his wife. Win, give back the winnings and you'll get what is currently the only sex scene she has.