Bowser has only one son?

[Goemar]

I feel like Zelda gets enough discussion, Metroid on the other hand while I don't play the games def. don't get nearly as much attention.

I think that's because Metroid has a more established time-line and most things seem to follow on pretty nicely (except Other M - but that's a spin-off so I don't count it).

I don't think it matters if Bowser's kids are "his" or not. They live in his castle and genetic or not they're his kids. Yeah, parenthood over blood and all that. Until something is said in-game though I'm not counting it as cannon.