I can't seem to get away from the F idea, it's not the most common of letters and only has one possible sound, so it's easy to notice and hard to forget that so far you've chosen F names. Other letters might be easier to ignore the link.
Maybe F in the middle of the name to tie the sibset together? I love Sofia.
I think another "ususual" letter at the beginning would be another way to not use F and have the sibset of names flow well together. W, Z, G, Sh, Sp, Sl, St, Sk T, Th, V for example. Zander, Gideon, Talia, Theodore, Theodora, Vana, Sloan, Skye are some ideas in my mind. Sorry I mixed boys and girls together.