The Suites at the All Star Music can accomodate up to 6 people, with a crib for someone under 3. Sounds like you'll have 7 people; so you have two options.
1) Look elsewhere
2) If one of your brother's children are under three, you're good to go
3) (And I know I'll get in trouble for saying this, so those of you who don't agree, ignore me) Get the suite anyways and bring an airmatress. BUT you wouldn't be able to use Extra Magic Hours because everyone needs to have proof they're staying in the room, and obviously, someone won't.
We've stayed 5 to a room in the All Star for as long as I can remember, and the space was never a problem.
That being said, of course it's dishonest, you're cheating Disney, and that may not be something you want to do.
If you can afford it, I think Slappy's idea of renting DVC points is great. Also, there are some hotels that fit five to a room. If you got adjoining rooms at, say, POR, you'd still have the two bathrooms, and there'd be a nice way to seperate the two families without making it feel like seperation.
HAVE FUN!!