First thing first, Nepal is only among those a few countries in the whole wide world that have something called "Relationship Certificate". If you guys have lived in the US long enough, have you supplies such US relationship certificate. It is only required in Nepal. US embassy doesn't give a damn about it. They care A LOT about birth certificate, which will have your parents info. That's about it.

So don't sweat it, dude. No reason to make it more complicated. Like someone said, their visit to AUS actually helps them.