Awful lot of generalisations on this thread.
I grew up as the elder of two and swore I would never have 2 children. IME it is the very worst number to have. When you have 2 and one is the favourite, the other is very obviously the unfavourite. We were very close as children but my DPs thought I should be responsible for my brother (2 years age gap) at all times. He was frequently a PITA but it was always my fault. This has pretty much carried on into adult life where nothing I do is ever good enough for my DM while DB pleases himself and she runs after him (bitter - you bet).
We had 4 very close together. DM frequently trots out the "poor little DD, all those brothers" and how we neglected her (pot, kettle) but we never ever ever made her responsible for any of her siblings. She did tend to get treated as older than she was, but not with responsibiities.
They missed out on stuff because we couldn't afford it, so one year they got a FP kitchen between them, but they were always grateful for what they got, unlike some of my friends children. DD did lots of activities but the DSs weren't interested. All had the same opportunities.
At one point they went to 4 different schools, and DH had an horrendous school run, but we were very careful to give each child what they needed and not just treat them as one lump. We worked shifts around them so they had both of us there equally, and the younger boys particularly saw quite a bit of their DGPs as they'd retired by then and were happy to do playgroup pickups.
Looking back there is lots I could have done better but we must have done something right. 2 of them live together and another lives with them both between travelling. They call and message each-other regularly - independently of me - and look out for each-other, and genuinely seem to really get along. We had everyone back for Christmas and they love all getting together.
We had DC5 when DC4 was 15. She is effectively an "only". She is in my face 24/7 demanding attention, and it is very wearing. I've managed to escape from her today but she's been in 4 times to see what I'm doing, because she can't play alone
. DH having been so hands on with the first 4 now works silly hours and has taken a big step back, so this is now just me and her.
It really comes down to the personality of each of the children, each of the parents and the dynamics within the family.