Oo lots of good suggestions given so far! I do like Odette, Mavis, Beryl and Etta of the above. Hmm how about Thea?
Mavis is a great suggestion! Love that.
Maisie or Marnie I think could also work.
I also think Regan (ree-gehn) would go well with your sibset.
Tamsin has the same feel for me as well.