I like the name
Felicity. My favorite of your combos is
Felicity Elizabeth, that sounds very sofisticated and feminine. While I also like the name
Frederick, I'm not feeling the whole alliteration thing. If there were other children in the family with names that didn't start with F I would say go for it, but there's something, at least to me, that's too 'picture perfect' about two sibs w/ the same first letter of their name.