OP, I can see why you're upset, that's a shitty thing to do - especially the not mentioning it. Sorry, but someone with an ounce of loyalty or care for you would have raised it - 'Are you really set on Dylan, we really want it too' - FGS she MUST have posted that thinking 'oooh wonder what Sis's face is going to look like when she sees the name'. Really, really shitty. You know where you stand with her now, more fool her...
However. You've had a lucky escape, honest. Dylan is ok, but it's just no way up there with a real classic name, or a properly unusual, beautiful name. It's a bit naff really. And yes, I've heard the 'Dildo' thing before too!
Post asking for suggestions. Henry is brilliant - can be Harry for short. Love Oscar and Keir from the ones suggested upthread. How about:
Gregory (Greg) - actually I quite like Grayson, but no good nickname, this could be a slightly better but similar name?
Magnus (Mags)
Oliver (Olly)
Otis
Felix
Edward (Ted, Teddy, Ned for short)
Personally I'd go for Magnus Henry and have him as Mags. That knocks Dylan James into a cocked hat!
I'm afraid I wouldn't be mature enough not to have a slightly catty reply ready if she raises the name thing - 'It's so strange, we'd started having doubts about it, especially when a friend mentioned that the one she knew got called Dildo in the playground! - but you know, it was when we saw it actually written down in your FB announcement that we both thought no, there's just not ENOUGH to that name, you know? Just a bit meh. Oh sorry, you know it is lovely and it really suits your Dylan of course...'