When it is my parents or sister, we ask for nothing for the room. Of course my parents always end up bringing gift cards and putting them towards our charges, since I often pay for meals in order to get the discounts (if I book anything that requires prepayment before the trip, I never ask them for the money. My parents do a lot for us, so we are happy to pay for some special meals or experiences). My dad always over estimates by quite a bit and ends up paying more than their share of meals, so I feel that it more than works out for all of us.
We brought my FIL for a once in a lifetime trip (he is disabled and the two nights away from home took a lot out of him physically) and paid for all his flights, food, everything. We just wanted him to have the experience with the kids. We asked for nothing from him, but he insisted on giving us a very substantial amount of money to put towards the trip for the kids to have fun with.
When friends came down with us in Sept, we didn't have enough points for the whole trip so they paid for the Ft Wilderness Cabins the first two nights. They are military, so they got great rates on them. They also got us discounted military tickets for our daughters, we paid for them but it saved us $400+(our girls didn't have annual passes since they weren't three the prior trip). In all I felt like it was pretty fair - they used us for our DVC and annual pass discounts, we used them for their military discounts, and we all had a great trip.
We have to book a 2BR for our family of 6, so for now it isn't a big deal to have others come along and use the lock off. When the kids are older and want more privacy, I can see us asking for help financially if friends want to come and we need to spring for a grand villa. But my parents and sister will always get a free ride. They save money all the time helping out with kids.