I actually have theories on this, regarding the mother(s) of the koopalings.
First, we have the possibility that all of them are from the same mother... which would be a simple explanation, but it would overlook the fact that Morton Jr. (the one with darker skin) has a different skin tone from his father and all of the other children. It is possible that all of them except for that one have the same mother.
Second, it is possible that each of them are only half-siblings to the other, although some of them look very much alike... leading me to believe that some of them share the same mother.
The theory I consider to be best is this: Bowser Jr. is some sort of clone, Ludwig and Larry (the two blue-haired ones) have the same mother, as do Wendy and Roy (the only ones with pink shells), and the same goes for Iggy and Lemmy (the ones with the rainbow-colored hair.) Morton Koopa Jr (brown one) would have have only half-siblings.
That would give us four mothers, and a clone. Possibly.
This is just a children's game series, of course. I REALLY don't think that Peach is the mother...
I only found out the names by looking them up on a wiki:
http://www.mariowiki.com/Koopalings