I’m assuming it’s just you in your house currently?
Having been to live with my parents for 18 months when my marriage broke down, and subsequently had my best mate (who is like family) living with me for six months, I’d make the following suggestions:
Is there enough space for them to have a separate area other than their bedroom to hang out? If not, is there facility for them to have TV in their room? Will you need a WiFi booster?
I think you should charge them a nominal amount of rent. You say you have no money…at a minimum you will no longer qualify for single person reduction on your Council Tax and your utility bills will increase, especially if your DIL is on May leave during the winter. Even if you stick it in an account and give all/most of it back to them when they leave, you should have a buffer.
Give some serious thought to kitchen arrangements…find out if your son and DIL like cooking from scratch. This was biggest cause of friction when I lived with my mum. I was allocated one cupboard, a shelf in the fridge and a drawer in the outside freezer and she was pedantic about enforcing that despite her and my dad living on ready meals whilst I always cook from scratch. It drove me absolutely nuts!
Deal with any issues calmly as and when they arise. There is nothing worse than working all week only for the weekend to be fucked over by bickering and arguments.
How is it going to work if you/they have friends round? I didn’t at all when living with my parents but we had an arrangement that DD’s (she was 11) best friend could come for tea/sleepovers every so often. When my friend lived with me she had not long started seeing someone and he was round a lot to the extent I had to lay down some ground rules which was a bit awkward.
What are they going to do with their stuff? Is there room for it at yours or will they need to pay for a storage facility? Be wary of the former…I still have my friend’s piano two years later 🙄.
I think what you’re doing is lovely and I would do the same in your shoes, but be under no illusions that it will be tough.