Why would you want to tell your niece that her mom is kicking her out?
Sort out a legal change of guardianship, get her covered on your health insurance (if needed in your area) and then tell her something like
You've been getting into a lot of trouble lately, but you seem to be doing better while you're here with me, and your mom wanted to make it ...
Having a LGBT+ family memeber, I can tell you that the people who care most aren't children. My brother is gay, and he'll never introduce himself as such especially to children. He'll just say "I'm a man" or "I'm a girl", so the child doesn't get confused/bewilded by unknown pronouns.
Also originality can be something valuable for a child.
The answer is incredibly simple:
You ask their parents. Period.
You're not their parent, so you don't need to get into complex discussions or judge their ability to participate in those discussions.
Besides, why stress-out about it. If you're worried the question will arise, simple ask their mum or dad how they want the question answered.
And another ...
A decent approach may be to keep it simple: "I'm still figuring that out", which sounds like a decent summary of where you are at the moment.
Most kids are pretty chill about adults admitting we don't know everything, and if they'd like more information, they generally have no problem asking follow-up questions.
If that's the case, it might be worth ...
Six years old is old enough to understand gender in a general sense, and it’s definitely old enough to have an intelligent conversation about the complexities of gender.
So my answer is to be honest with them and tell them how you feel. If you know, then tell them. If you’re not sure, then say so- and explain why, and what is going on in your head. Say ...