Why do you think Mattie wasn't at Silas doing her mother's bidding the way Will and Carm were? it seems like she keeps her kids on a short leash. except for mattie.
Honestly, I kind of read Mattie as the un-favorite among her "children." Maybe she was the least biddable, or at least the hardest to keep in line.
I'd say there were racial undertones to the Black vampire being the one her "mother" liked the least- except that (while we never see Lilita's original body before she possesses Perry) the mother in question is presumably not white herself, being Sumerian. Also she's not human- she's a goddess. So even though racism among people of color is of course a Thing, I suspect that doesn't factor into it.
Then I'm back to "Mattie has an uneasy detente with Lilita because she's the one who's old enough or canny enough to see through her manipulation and who has nothing left that Lilita can really hold over her without hurting the cause." She doesn't give a damn about Will; she doesn't form bonds with humans. Any loved ones she had from her mortal life have been dead for centuries. And eliminating Carmilla permanently would inconvenience Lilita's plan, which Mattie is probably well aware of.
Carm is a psychologically broken nihilist who's given up resisting, at least at the beginning of s1. Will is a devoted lackey. But Mattie...Mattie, she can't control, and yet still needs as a piece on her chessboard.
That leaves Mattie off doing whatever she damn well pleases until she has to come back and do something with Corve- which benefits her personally and lets her check in on her sister, or I doubt she'd even bother with that.