Many people think that Kelly is Trubel's mom. I, frankly, really really hope not, because if Kelly is Trubel's mom, then I can no longer have any respect for Kelly whatsoever.
Think about it... one of the few things that's known about Grimm physiology is that female Grimms typically come into their abilities much earlier than male Grimms do. During childhood, in fact. Kelly was, I believe, 11 when she manifested as a Grimm, and I think Trubel was 10.
So. Hypothetical. You're Kelly. You're a Grimm. You know perfectly well that female Grimms typically manifest in childhood. You have just given birth to a baby girl. And you put her in foster care?
At least Nick was a) a boy, and therefore unlikely to manifest until adulthood, and b) left in the care of his aunt, who, worst case scenario, could at least explain things to him. Trubel was left alone in a world where people turn into monsters that nobody else can see that then try to kill you for no apparent reason, as a child.
The only excuse for any parent doing that to their daughter is if they died. And Kelly didn't die when Trubel was a child. Therefore, either Kelly is not Trubel's mother, or she is an irredeemable psychopath. I prefer the first possibility. :)