@Kyonne
Well, what is wrong with that. As far as they know, they already could have done worse then just kissing in private at home (in the worst case sex at least) and it is not like they are going to hurt eachother purposefully or one of them is forcing the other since both admitted to wanting it. Plus it is a lot to think about which will take time and having to babysit them so they do not meet is kind of pointless now anyways. Plus it is not like they were entirely clueless. They knew they were very close, just not that they were very close in a romantic way.
Also, they are not dumb children, so their parents should know that their kids know about the negatives of inbreeding and likely would never attempt it, especially since Uta being pregnant would be obvious to anyone that something was up and they were somewhat trying their best to not get found out, plus the whole underage thing. So leaving them alone for the night should be fine because they likely wont do much worse than what they saw.