Susannah and
Frederick sound like a lovely sib-set.
I think I slightly prefer the spelling
Susanna.
I don't really like
Annie as a NN for this, though I do adore
Anna on its own.
Maybe Sannah or
Zanna could work as a nickname, if you don't like Suzy/Susie? Or even
Lily, as in the meaning of the name.
(vote on my list?
http://www.babynames.com/namelist/9516801)