I think that some behaviours are more age-related or related to family dynamics in general than to being an only child.

My brother's step kids are 5 & 7 and they don't share very well at all, and are always competing with my kids for attention and are generally very difficult to spend time with. I don't think that kids learn how to be around other people because they have siblings. I think they learn because adults take the time to show them ... and sometimes they are just going through a phase.

I think lots of behaviours get assigned as 'typical' of an only child, eldest child, youngest child, middle child etc but I think a lot of the time it is more to do with how the adults around them operate. I am not a huge fan of the stereotypes and don't like the idea of kids being pigeon holed because of their birth order or only child status. I think there are circumstances that lead to certain behaviours developing, but it doesn't have to be like that and parents can put in a lot of effort to ensure some of those issues aren't issue, iykwim.

All relationships take work - whether you have one kid or have ten kids - it takes a lot of work and effort to make sure that everyone is having their needs met. I think the best that parents can do for their kids isn't to ensure they have a sibling but rather ensure their needs of their all their kids, no matter how few or many they have, are met while also meeting their own needs. Obviously, there are going to be different needs in different families but it depends on the dynamics.