I told DD something similar too. She seemed to think that sounded alright. Not having a sister myself (or brothers for that matter, but that's a different story), I tend to talk to my mom a lot so hopefully my DD and I will have a close relationship too.
I forgot to mention, see the "floaties" in the gender shot pic? That's his cord and it was all over the place and going between his legs. I won't lie that I'm a tiny but nervous it could've got in the way and made a girl look like a boy but hopefully by enough to fool us twice since she came back and looked again and still said boy although the shot was even more difficult to see for me.