J.T.
Medication may help, but so will some ground rules.
Make sure the your sisters and thier kids know the rules about sharing and putting toys away when they are done playing. Put some extra garbage pails around so they have no excuse for not using one, and if you see them drop something on the floor, ask them to please pick it up and throw it away. Really, at that age it is more about educating your sister about what the kids should be doing and then all of the adults should enforce it. At that age kids do just put things down, not becuase htey are bad, but because they really don't think about / remember that they should be throwing it out...
If your BIL getting drunk is an issue, no more alcohol. You do not need your kids thinking that is acceptable behavior anyway (never mind the mess he makes, which would give me anxiety on its own...eewww).
For your 3 year old, make sure she has a quiet place to go, where no one else can be so she can get away of she needs to that (or you can direct her there for some quiet time).
Good luck!