Its just pot luck. Some kids like being an only, some hate it. Some love their siblings and some can't stand theirs!
Fwiw I am an only and growing up I hated it and was v.jealous of the other children who had siblings. The one thing that helped me was I had a cousin who I was extremely close too (basically like a sister) and other cousins similar age.
When I met my partner I suddenly realised that I wasn't so unlucky after all, He doesn't have a great relationship with either of his brothers (similar age) and theres a big age gap between him and sister which made it a bit difficult. Not so much now though :)
My mum also has 4 siblings and no contact with any. A couple are actually vile, honestly siblings are not the be all and end all as I grew to find out.
I have come to terms happily now I am older. I have my own family my own child and somehow having a sibling seems so much less important than it did in childhood. The only issue I have now is I will never be an auntie, which I will just have to get over eventually.
I was a miracle baby after a still born, I was suspected to also be another still born so I know it wasn't realistic for my mum to have another child. I don't resent my parents at all and I had a fantastic childhood.
Along as she does have other children to play with it won't matter whether they're brothers, sisters, friends etc.
Your dd will be fine but its really nice that you're thinking about ways to make her feel better about only childhood. I don't think you have anything to worry about, she may love it, it does have its perks!