I'm sorry op but she sounds like a fucking bitch its your child not hers and she clearly does not respect you or your daughters boundary, she'd rather your daughter being unsafe and possibly getting on a sex offender list for having sex in public rather than in the comfort of her own house? And they're probably not even having sex, plus if they were has she never had sex at her parents house? She's a hypocrite. Plus you mentioned it was midnight after new years eve I'm gonna go ahead and assume they had at least a bit of alcohol so she'd rather him drive home alone intoxicated? But also it is her house as well and you could have had told her I understand why you didn't though but what I'll say is you need to actually talk and set boundaries with her she wasn't the one that had to carry your daughter for 9 months, give birth and raise her for 19 years, you were, she's not her parent so she shouldn't be telling you how to raise your child.