My DD moved back in with me and her brother for 18 months, bringing her DH and their 2 year old.
We only have one bathroom so it was a bit cosy sometimes but we made it work by figuring out timings depending on who had to be out when. We mostly shopped separately except for things like loo roll and had a WhatsApp group in case anyone needed a parcel/sundries picked up or had forgotten their key!
We didn't spend huge amounts of time all together as I work shifts, and I added a small sofa to my bedroom so that I could retreat alone or with friends.
The only house rule I insisted upon was a clear sink at all times, my kitchen is too small to leave dishes for hours after meals.
I babysat occasionally but there was no expectation, we 'parented' collectively but he is their child. I loved being able to see him grow every day 
I'd imagine that if your DIL has any issues, she may channel them through your DS rather than direct with you but that depends on your relationship with both of them.
I wish you well, it's a lovely thing to do for them.