Another thing to think about since it's been several years since you've seen any of them...she may be a smart whipper snapper and be able to figure out the sham of the JW religion on her own. She may be ahead of you. And if that's the case, she'll contact you when she's ready (if she knows that you're an apostate).
But even if she is fully indoctrinated and buying into it as much as her parents, you can't beat yourself up over it. In the end, she is your brother's child and he is ultimately responsible for how she is raised.
What if you make contact when she's a legal adult and she has bought into the whole idea of 'hating the apostate' and tells you to leave her alone? Then you've spent all this time worrying about someone who doesn't want your help.
It's tough, but we can't help every single person, family or not, that is caught up in this religion. We have to do what we can when we can and not worry about what we have no control over. And at this point in time, you have no control over this situation.
disclaimer: throw all that out the window if it comes down to an emergency, like a blood transfusion. That changes everything and now a life is on the line.