If you really like Shae (or spelling variations of) then go with it.. it IS a middle name and I'm sure nobody will really mind. I named my kids Katie and Ethan when I had two cousins named Kayley and Ian.
My SIL wanted to name her new baby boy Ethan too, I didn't know this when I named my son but I really wouldn't have cared if they shared the same name. Mine would have been Ethan Jacob and her's Ethan Jude... a name is a name.. it's a big deal that YOU like it, not so much what others think.
I like Shae-sounds very nice