I'm not a huge fan of siblings all sharing same letters, or even sounds, but
Sage,
Cyrus and
Sibyl aren't that bad, especially when you've got the C spelling in there to shake it up.
I prefer the spelling
Sybil, but not with a brother named
Cyrus.
Sibyl works better.
I do like
Sybil /
Sibyl. I've liked it for a while, but have to admit my opinion went up when the nicest, most admirable daughter in Downton
Abbey was named that.
Sybilla or
Sibylla are nice too, though a touch more Germanic sounding.
Sibyl Alexandra is about the only combo I can think of right now. Maybe
Sibyl Marguerite?