A few posts below is a link to a guide to the whole game - by me take a look there if you are stuck somewhere.
If you have done the mission correctly, after entering your parents' room at night he should wake up - a green sig should automatically appear next to him - this is the wine your father drinks before going to sleep - then in the bathroom - there is a cupboard - you use the power as I remember unlock and then telekinesis (you are too small to reach it) you take sleeping pills - in the evening you go to the room and add the pills to the wine - you repeat the sleeping actions all the time, that is, you get the pills, add them to the wine, after which you go back to your mother at night.
The important thing is that after using the power of Lust 2 times you have to perform the mission so that your father no sleep - the first is to unlock a new mission (scene is father sex with mom), the second is to see an additional scene (this you perform when you complete the mission that unlocked - its name is Mom's Flustration (scene is father masturbate with new sex clothes mom))
Mom's frustration: You will complete the first stage without any problems - but to convince your mother to buy new underwear, you must repeat the first stage as many as 3 times. The difference is in the scenes you should see 1 scene then 1 scene and 2 and finally 1,2 scene and 3.