What if:
There was a set timeline that was going to happen. You could choose to play as the father of the household or the son and the summer would pass as set out in the script. A family of six, father, mother, eldest son, and three younger daughters/sisters. The main character, depending on who they are, has a limited supply of pills to put everyone to sleep and a perfect delivery method to do it (The water filter). This way, as the summer progresses, relatives come over and the main character gets to pick from any of his siblings, cousins, aunts, or female friends that happen to come over during that time. There could be a sleep over, cousins coming to visit, or just normal incest with his sisters/daughters.
This way, not all experiences can be had at once with the limited number of times everyone will be asleep. What about your uncle getting married? All of the bride's maids that would be sleeping in your large house, or even the bride to be?
Just ideas. Pregnancy would be good ;)