I dislike both
Sidney and
Sydney for girls; I know a
Jennifer Sidney whose second name honours her father, and it's fine for that reason and as a mn.
Sidonia I suppose looks slightly fem with that -a ending; all I can think of is the unfortunate
Duke of
Medina Sidonia who was ordered to command the Spanish invasion fleet against the English in 1588. He had no experience at all, did his best, but the Armada lost.