You need to get to the point where you can have sex with her in the pool, get the special sex scene in bed, usually on day six, or beat her at the videogames in her underwear, and tell her to remove them, but then don't have sex with her the first couple times you do that, but instead get a blowjob, then move to getting her to spread her legs, and bust a nut on her, then,the third time you usually can get her to have sex. Once you succeed in having sex in any way, she usually will remove her underwear on request.
With version 0.91.3 having some of the issues with missing points from actions fixed, it is very easy to get to where she will remove her underwear in the dress-less videogame by day 3 or 4, since now with the vedor working at the fountain everyday, it isn't difficult at all to get at least 150 points every day, plus a lot of stat boosts to exposure, lust, etc. by the middle of day 7, I am above 900 RWC, with 22 peep, 35 lust, 12 jerking off, 17 exposure, 20 touch, 86 happiness, and I still have 68 energy left on this game save to continue to do stuff. On this game save, I still have available the option to have sex with her in the pool, the front room, Vanilla's room, use the carrot toy on her, all sorts of other things. This is where I am at after I came back from downtown, had sex at the movies, and on the motorbike ride. Everyone's concerns about needing to jack up the stats on this can be laid to rest now.