You can't force siblings, blood or not, to be friends. My sister and I were at each other's throats sometimes when we were kids, but at least our parents didn't force us to hang out with each other all the time. That definitely would've made it worse and could've made it so we couldn't have become friends as adults, which we ultimately did. Sometimes, siblings just don't get along, and the worst thing you can do when that happens is place the blame on one of the siblings for not being a good enough friend or not including their sister or brother enough. I know it's tough as a parent to watch one sibling have a lot of friends when the other can't find a single playmate, but forcing the sibling with friends to include the other won't solve the underlying problem. If left unsolved for too long, it could become a situation like this.