Thanks everybody! My SIL IS a bit...well, annoyable I guess. I just don't want to piss her off. But you're right, I'm probably over thinking it. This is just the first girl name we have both really liked, and I like Immy as a nickname...but that certainly won't work because her daughter is called Emmy sometimes. But there are lots of nicknames for Imogen, and really I like it without a nickname. Thanks for all the input! I feel better about it, and it is definitely our girl name, if we have a girl. Thanks again!
Also, any ideas for middle names? I like Louisa, and Louise is my sister's middle name, so it would be a sort of in honor of her thing.