I've seen a massive family where I suspect some of the kids to be on the spectrum.
The mother used to tell me she had lots of autistic traits. (That was before I realised and found out I was on the spectrum). She's got plenty of spectrum family relatives, both severe and more mild. The big, very structured, family does work well. Somehow for all the kids, there is a kind of safety in numbers. And, they are valued, no matter how they are. All kids are welcome, none is seen as defective. There is less individual attention possible though. And a lot less personal space too.
For my oldest, it was a good thing to get a little sister. Now she's a teen, she's not always that happy with her younger sister. Guess that that is true for a lot of teens. But, her sister did help her find courage for a lot of things.
I mostly loved having siblings.
Like my oldest, I did find courage because of the brother under me being more daring, and taking me with him.
And now, I love to talk to my second brother, who is very much like me.
Would I have been unhappy, or off worse had I been on my own? Don't think so.
On the other hand, depression seems to happen more when you have no close relationships.
Having rotten relationships with siblings, because of being the only one in the family on the spectrum would not be helpful though.