I don't thing the cabins at fort wilderness would work well for you. They only have one bedroom. You and your mom would have to share full sized bed and the kids could be in the bunk beds in the bedroom, but your brother and his wife would have to sleep on the murphy bed in the family room. Not really the privacy I would think they would want.
What kind of "kitchenette" is he looking for? Is a microwave, fridge and sink enough? The family suites at the values have that. He can book that and you can book a regular room this way the extra price only falls on them. I think once he compares the price of a regular room to a family suite he might think that just having a refrigerator is enough. It's one thing to spend more money on the suite when you have kids, but for that price for two people I'd go to a higher end resort (YMMV).
FYI, by the time he books a family suite and you book a regular one, you have almost paid for a 2 bedroom villa at OKW or SSR. If you can rent points from a DVC member you can probably get the villa cheaper than you can rent two rooms. Don't forget the tree house villas either, they are about the price of a two bedroom (and the same size), but they are configured differently and have 3 bedrooms. Remember that if you don't want to rent points and you book through Disney, the discounts of the villas are usually much higher than the discounts on the values (15% vs 30%) so if you go during a time that usually offers discounts take that into consideration.
If you want budget though as you mentioned in your OP, then two standard rooms at a value resort will give him the couple their privacy while being the cheapest option.