"have another so she'll have someone to play with"
Currently at the park with my only. She's happily playing by herself, climbing, going down slides, running around, occasionally pretend play and bringing me "food" from her store- but mostly entertaining herself.
Meanwhile, a dad is here with his two kids, pretty close in age, I'd guess 5-7. One kid wants to be pushed on the swing, one kid is yelling at Dad to come play a game with him on the playground. A few minutes ago, one wanted to ride his scooter around the park but the other wanted to climb... They do not have any interest in playing together and dad is struggling...
I know this is a smalllllll tiny window into their day, I'm sure the kids do frequently play together, buttttt I think it's also helpful to see/know that siblings are not automatic play mates.