I've met two Elsas (one in her early twenties, the other in her late forties), and I don't think it's old-fashioned at all. It's a nice name, but not one I'd ever consider for a future daughter; it doesn't appeal to me that much. I like the family significance it holds in your case, though.
I don't like
Noah for a girl (only
Noa - weird, I know), but the combo is nice enough. As for the sibset, I think
Estella (which I really like) and
Elsa Noelle are too similar sound-wise. Have you considered
Estrella rather than
Estella?