One question: does the mother have sex with her husband while the kids are in the house? If so, she's a tad hipocritical... Would she like it if her daughter were to walk into the bedroom while she's boinking? I think not. IMHO she should accept that the daughter is having sex, even though the rules say she shouldn't. Rules are there to be broken, and if the rule is unrealistic, it should be changed.
At least that's the way I was brought up... Perhaps I'm too reasonable.
And of course, I agree with the posters that point at the other options: cheap motel, backseat of car. I'd rather have my kids doing "it" in a safe environment, with a supply of condoms around.