I never did this (I wasn't pregnant during Halloween) but I have seen a few couples who have and some get super creative!! I really like what your SIL and brother did, it's cute.
I thought about it last year, and bought a maternity devil outfit, but it didn't really fit right, they were one size fits all, but apparently one size means small boobs, lol.
Very cute, I've heard of a couple of beer gut pics that would actually be pretty fun too.
I've never dressed up, but a friend of mine's husband went as a woman, and she carried around a beer, dressed as a man in overalls with his "beer gut" hanging out, lol.