Jellyfish - I'm not an only child, but I won't "have" anyone when my parents die because I barely know my two brothers! We're like strangers when we meet - it's all very awkward.
Does your DD have any extended family? My DD has 3 cousins around the same age, plus more to come shortly

You're right, she'll work it out. I know SO many people who don't get along with their adult siblings, so they're not in contact anyway. Having a brother or sister isn't a guarantee of a close family.... I know what you mean though.