SPOILERS!
I added these two answers to my FAQ:
31) How do I have sex with Yvette?
There are many ways to have sex with Yvette. The first step is to raise her stats of affection, love, and lust. The easiest way to get her to have sex with you is to let her win the Best Butt or Wet T-shirt contest. If she makes her affection check, she'll reward you with sex in her bedroom. You can also get this reward for her strength training, but this way the event is repeatable. Make sure that your endurance is high to not just stop at a blowjob. Also, she may come to your room at 3am after having a nightmare. Here, if you don't tell her that her lingerie has slipped, and get caught jacking off next to her, and she makes her lust check, she'll have sex with you, cowgirl style. You can also meet her when she is working reception at midnight. If her lust is high, and you compliment her, and your endurance is high enough so that you don't just jack off all over her, she'll have sex with you on the desk. If you catch her masturbating, and call out to her instead of sneaking around, and are nice to her, and have an erection, she'll realize that she doesn't have to masturbate when she can have sex with you. Lastly, after she comes back from visiting her sister, she'll reward you with dinner and have sex with you afterward.
32) How do I have dinner with Yvette? Is it repeatable?
Yvette will come to your door crying that she needs to go home to take care of her sister who has had an emergency. After you send her home for at least a week, you can invite her to come back. She will reward you with a private dinner. She will tell you about her first boyfriend. After dinner, she will take off her panties. When she comes back, she will have sex with you. You can repeat this event to hear more about her or tell her about your first girlfriend by letting her win the Best Butt or Wet T-shirt contest. Then she will reward you either with the sex scene in her room or the dinner scene, a 50/50 chance.