For what it's worth I am 1 of 3 girls. My elder sister I have nothing in common with, I love and care about her but we are not friends. She has always been a bit of a whiny brat and takes rather than gives.
I'm great friends with my younger sister. Always have been.
However, I think my friendship with my younger sister stopped me focusing on making friends with other people as a child and I find friendships difficult as an adult (maybe that is more to do with the fact my parents never socialised)
A PP commented about rivalries/siblings being labelled. I get that, I was always the sensible/clever one and wow did it put the pressure on to never fuck up. My younger sister was always the pretty one, which made me feel ugly. My Older sister was the favourite and got away with just being horrible to us...she still is, then acts like a victim of life which I have zero time for.
I am massively aware of the pressure having 3 kids put on my parents, their relationship and their finances. I don't want to spend the next 10-20 years arguing with the man I love because we are both exhausted and skint.
The gap my DD would have with a sibling is a worry. They would always be at totally different stages in life. I have found the hardest thing about parenting being how as soon as you discover the rules the game changes and you have to work it out all over again. at almost 10 she needs me less physically but her emotional need is through the roof as she tries to understand the world. I don't think I could switch effectively or efficiently between the physical demands of a toddler and the emotional needs of a teenager.