I like
Xavier and
Xander, but Xane and Xoë seem contrived to me. I prefer
Zane and Zoë. I don't really see why they felt compelled to give all of their kids names that begin with the same letter. It seems unnecessary to me, and changing the spellings of names just to accommodate the theme just seems like completely overdoing it.
Xanthe is lovely. I think it's a bit too close to
Xander to use in the same sibset, but in general, I agree that it's preferable to Xoë.