INCORRECT EMMANINJA! In the 2014 comic con, the producer person said that mutagen can mutate any LIVING thing together! The turtles were mutated on the ground. they couldn't mutate into the ground or air. the only thing they could mutate was with splinter. they only mutated with his living-ness.
"GIRL YOU GOT ME AKIN' SO CRAY CRAY!"~Sev'ral times. (Gravity Falls)
Get ready for a rant here. *takes a deep breath*
Splinter's DNA was the last thing to touch the boys, his blueprints created the turtles. The boys have three blood parents, oddly enough. There mother and father, then there's Splinter. So shipping Leorai would be Incest.. and plus, he doesn't have a thing for her any more from what I want to believe. ;A; I don't know, that's just my opinion. I see more of a sibling relationship.
The turtles are also biological brothers, they are from the same clutch of eggs, and also have the same parents. Also just my opinion. Sorry if I hurt anyone's feelings. ;___;
and even if emmaninjas theory is incorrect (no offense; I agree), they may be from the same brood, since it is neither confirmed nor denied. does it really matter, though? they lived most of their life together considering each other brothers. they have the relationship of brothers, and that's what counts.