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.
But that's the thing. I don't think they were that close! Or rather, I don't think Moonshade and Bearclaw were that close. Bearclaw might have elected to not even tell Cutter that Moonshade is his aunt. Also, didn't Bearclaw take it rather very-much-personal when Crescent was killed? As in; a little more personal than simply "I'm chief, and we're so few already!"
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