Board Thread:General Discussion/@comment-31231137-20170728195559/@comment-33151151-20170807185410

Cooler1991 wrote: No, because Jeffy is a human and Bowser Junior is a koopaling. They are not biologically related, but both Bowser Junior and Jeffy were adopted. Bowser Junior was adopted by Bowser and Jeffy was forcibly adopted by Mario. Junior was only mentioned as adoptive in SuperPowers 2 and MLSDA, which means that it may not be canon to the SML Timeline, his name is BOWSER Junior (or Robert Bernard Junior in The Koopa Kids), and I'm pretty sure that the father must be named Bowser in order to do that. Junior has always and will be Bowser's biological son.

On the other hand, Jeffy's adoption was canon and his biological father is Francois, who is NOT related to Bowser.

Even if it's officially confirmed that he is in fact adopted, he and Jeffy will never be related (unless gay marriage comes or some shit like that).