I have to admit the first thing that came to mind was Xylophone, what were you considering before?
I love [name]Isla[/name] [name]Francesca[/name] [name]Belle[/name] but I agree [name]Isla[/name] is bound for popularity, some other less popular alternative to [name]Isla[/name] could be [name]Ida[/name], [name]Ira[/name], [name]Lyra[/name], [name]Shyla[/name], [name]Ayla[/name], [name]Willa[/name], or maybe even [name]Zelia[/name]
Or maybe to tie your son and daughters names you could use a name that contains an X like… [name]Roxane[/name], [name]Trixie[/name], [name]Dixie[/name], [name]Beatrix[/name], [name]Arantxa[/name], Eudoxia, [name]Margaux[/name], [name]Roxie[/name]…
Or maybe just a name that contains one of those unexpected letters like x, y, z, w, v, etc. like… Azelie, [name]Azalea[/name], [name]Azariah[/name], [name]Carys[/name], [name]Hazel[/name], [name]Izara[/name], [name]Keziah[/name], [name]Maelys[/name], [name]Rowan[/name], [name]Riva[/name], [name]Quarry[/name], [name]Silvia[/name], [name]Soraya[/name], [name]Suvi[/name], [name]Terezie[/name], [name]Tinsley[/name], [name]Valentine[/name], [name]Verity[/name], [name]Waverly[/name], [name]Willow[/name], [name]Willa[/name], [name]Yara[/name], Zinniah…
Personally I think [name]Silvia[/name] & [name]Xander[/name] would be an adorable sibset…