I think what frostbite's trying to say, is that Morton Sr. is either Bowser's cousin or brother.
I personally don't believe that Morton is even related to Bowser at all, probably just like Morton Smith Jr, then Bowser goes into the orphanage, points to Morton Smith Jr, and says "I want that one!"
But hey, we shouldn't question Dark Land's great ruler.
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*Blows horn*
"Please make way, for the greatest, most stupendest person of all, the one the only, Danorek Benjamin Weiless III!"
"Oh, shut up!"