BlackWaveComing's advice seems excellent - sound and really age appropriate.
The difference between sharing that you are T rather than LGB to a child is that T insists on rewriting everyone's history. It's not 'this is the type of person I happen to fancy' - the answer to which generally from a child is a shrug of the shoulders and 'whatever' because sexual attraction isn't really a thing for them yet.
Rather, in its current iteration, it's 'You thought I was this one thing but you were wrong, I am not. Now you all need to agree with me that I was this new thing all along.'
Activists might like us to think that the response to the T should also be a shrug and a 'whatever' too. Yet while children have a lot of magical thinking at that age and so might instinctively do that, still, as parents, and particularly parents of girls, we can see that, heading down the track, there are big issues - especially with regard to girls' boundaries and consent - where it really does matter ultimately what sex people are.
This would be far more obvious if the scenario was a nephew rather than a niece with a trans identity and your girls were being asked to consider a male as being just like them.
The other thing to consider is your 'debrief' from the meeting because there will be a certain amount of treading on eggshells while there, which probably will be exhausting for all.
Politeness on the visit would be to try to remember the new name and preferred pronouns as indicated by DN, along with 'all our other manners'.
On the journey home I would revert as a family to the previous name and sex based pronouns and continue to use them in your own home because that gives your girls permission to speak freely and explore their own feelings in the real world.
Compelled speech is not kindness and it doesn't have to extend to my car. You, for now, are the keeper of reality for your little children. Good luck!