I like Erin no matter what, it was my top pick for a girl. I think Catherine is my fav middle name choice. Although they both have -in endings which kind of make them sound like a rhyme. But I still like it as long as your last name doesn't end with the -in sound.
minimunkle: Genevieve is the top choice for us for a middle name. Aria Genevieve. But there are a few others on that list too so we will have to see what she looks like.
Do you know the Sarah McLachlan/Delirium song called Silence? Well, the chorus talks about "in this white wave I am sinking in this silence..." great song and so I love that Genevieve means white wave. You should check it out!
My last name also starts with an S but I agree that if it isn't with a SH sound you are fine. :-) Happy choosing!