DD, SIL, and his 5yo son moved in with us in March. We have a very large house and until they joined us it was just DH and me here. They have 2 bedrooms, their own bathroom, and their own living room. We share the kitchen, dining room, laundry area, and workout area in the basement (it's NOT fancy, it's an elliptical, treadmill, and exercise bike in what is basically a storage room). DD went to grad school last fall and although she has an assistantship, it doesn't provide a very large stipend. SIL has a full-time job with good benefits but not such terrific pay, and there are enormous medical bills and legal bills that he has to deal with, so they were really strapped financially. The 5yo is adorable and smart but he has some issues that effect his behavior, which is sometimes a challenge. For the most part, it's been a good arrangement. DH and I are currently paying for everything but their food, but really the only thing that's gone up is the electric bill, and they have offered to kick in some money on that (but we are OK right now, we know how strapped they are and DD won't have a summer stipend, so...). We are 4 adults sharing 3 vehicles, and whomever is driving when a car needs gas is responsible for putting some gas in the tank. Doesn't have to be full, but NOBODY wants to have to stop on the way to work in the morning or be stranded! ANYHOW... they are responsible for cleaning their own rooms and are pretty good about that. We have some kitchen issues, mostly around how dishes are done (and some of that is squarely on me) but we are working this out. They had to learn that when we say something, we are not being mean, critical, or bossy, and we had to learn that they live with different standards than we do. We have come to an agreement and the kitchen is cleaned every night, and really it is working out with minimal grumbling. As far as babysitting, they always ask if we mind if they go out, and it's always after The Boy is in bed; usually they walk into town for a beer or two, we work at home in the evenings anyhow, The Boy NEVER wakes up, and everything in town closes by 11pm, so it's a pretty controlled situation. DD and SIL have only been married since October. We are happy to be able to do this for them as they are still learning to be a couple together, to be newly-weds, to grow together. That's hard with a 5 year old in the picture, and SIL is a pretty good father. We help with after school pick up as the 5yo is no longer welcomed in aftercare, and we all have a vested interest in SIL being able to keep his job. Usually I do one day (today, as a matter of fact), DH does one day, SIL works from home on Wednesdays, and DD or SIL's parents juggle the other two days. It helps that DH and I both work in academia and can be fairly flexible with our schedules, especially now that it's summer. Pick up is by 3pm and SIL is home by 5pm, so it's not bad. The Boy is pretty self-entertaining and loves to play in the backyard now that it's nice out. I usually sit on the deck and keep an eye on him while grading or something, or we'll go grocery shopping or to the park, etc. Once again, not too stressful, and never expected; SIL checks every week to see if we are available, and on what day(s) and if necessary, he'll take a couple of hours of PTO to do the after school care. Regarding food, we mostly buy our groceries separately (they have peanut allergies, egg allergies, dairy intolerance, etc to deal with), but we share leftovers most of the time. If one of us is making something "special" we'll invite the others; we usually end up sharing meals maybe twice a week. The Boy eats by 6 and is in bed by 7-ish, after which the adults cook/eat, so he often has "Monday leftovers" for dinner on Tuesday, for example, which minimizes the kitchen mess and makes it easier on whomever is doing his meal prep.
Overall... we are thrilled to have them with us! My DD is the person in the world to whom I am closest, and I love that she's down the hall instead of a 6 hour drive away. We've known SIL for 8 years, back since the two of them platonically shared an apartment for senior year of college. It's a 2nd marriage for both, each of them having had terrible experiences the first time around. We want them to finish their educations so they can have careers that pay well, not just hourly jobs with questionable wages, so they can eventually buy their own home, raise a family, and have an easier life than they've had so far.
Honestly the person I have to watch on that is DW. Like most moms, her instinct is to want to mother her little boy even though he's all grown up, and she loves taking care of the little one. And they are happy to let her.
