Okay, and another thing about titles is that they are handed DOWN, never up the family tree. If Hugh died and Stephen inherited, Allan could never inherit after that, even if Stephen died without sons. The title would go to a collateral line. If Stephen died and then Hugh died without other sons, then he Would inherit. So not only birth order, but death order is critical to the inheritance structure.
I think that since Allan's father didn't disinherit him, his brother can't. Can siblings disinherit? I haven't found any similar examples so I'm not actually sure.