I think it would be unfair of them to expect you to take them in under these circumstances. As a parent, the last thing I want to do is be a burden on my kids. It would be one thing if they were too sick or old to help themselves but to expect you to foot the bill while they choose not to have an income is too much to expect.
There is a scripture at Thessalonians 3:10 that says basically that if you don't work....you don't eat. By Pioneering, they are choosing not to spend their time engaging in income generating work and should not expect others to suport them. In my area, in order to obey Cesar's law, one who is receving unemployment benefits must be actively persuing employment and is not allowed to turn down suitable work.
Once you let them move in, it would be difficult to get them out without a huge family blowout. If they have their own home, it's likely they could stay there less expensively than it would be to rent an apartment. Maybe they should look for another Pioneer couple or Border to move in with them and they can charge rent.