My IL's gave us 10K towards our down payment but it was something they did for all their children using the money made from the sale of their childhood home.
In your situation I don't think I could do it. It seems kind of like a fool me once situation. They have already tried to control and now that they can't they need a new way in and it seems like they know a house is somewhat of a "weakness" so to speak
((((Hugs)))). Sorry for everything you have been through.