it sounds a lil bit weak to me, honestly.
I'm pretty much convinced that if they were really that close (Cutter and Moonshade, I mean) we would have known.
After all, kinships have been always stated, in the story...maybe with the exception of Nightfall and Scouter being cousins, but I'm pretty much sure that was a later add, cause the only hint at nightfall's parents in comics comes from "Wolfrider", and she even has different parents in the novels,which leads me to think that Nightfall's parents never had a name before that Wolfrider! issue... And so, Scouter and Nightfall became cousins only later in the story, and that's why their kinship has never been addressed during all the archs that came before Wolfrider.
So..that's why I think that, while there is no concrete clue against this particular theory, I see it as rather implausible. After all, there is no concrete clue in favour of it, considering that not everything in the novels is to be considered canon...and that there's nothing implying a kinship between Bearclaw and Moonshade in canon.
You know, considering that Bearclaw was some sort of adoptive father to Strongbow...I do think that any kinship between Moonshade and Bearclaw wouldn't have gone missing xD