That's the thing about fanon you gotta do your research on cultures they're based on. I went on the premise that because they're nomads, they shouldn't "marry" each other because of their free lifestyle. Also based on research ancient tibetan monks never married, the ancient tibetan people's also had fertility festivals and were commonly polyamorous and polysexual.

I never heard of nothing like this.

Aang probably did have siblings, but the air nomads were raised communally, so I doubt he would have known who they were. plus the air temples were gender segregated