Thanks everybody for your replies.
They are not homeless. They live with my wife's brother. This is in China a relatively normal custom, children take care of the parents, parents look after the grandchildren. No need for a retirement home, no need for childcare, but unfortunantly also no use for privacy
But that is in that sense not under discussion now, it's just to sketch the common habit.
Regarding other people taking them, no, nobody around. We live 23 hours by train from their home and are the only ones in this surrounding.
My wife understands my opinions and is also not thrilled to have them over (although she is (luckily) not raised traditionally so that she has to listen to what I say). We have a rather good idea of eachothers expectations and values. She has a relatively western way of thinking, due to our time together and her staying in the west. It is just that when being on the phone, in order to make it easier for them, acting out of her education, the invitation was already out before she could reconsider. We also stayed with her brother and her parents, no problem to them (although it was only a few days and really to visit them as family, which is different from this case, being further away family and for a longer time). And rejecting them would be an unacceptable loss of face.
I think I'll have to accept the situation as it comes and try to live by the maxim Anna mentioned: "This too will pass...". And make sure I have tape ready to silence my wife next time her education pops up