Proverbs 18:24
A man who has friends must himself be friendly,
But there is a friend who sticks closer than a brother.

You cannot choose your family but you can choose your friends.
If you are friends with someone, you chose to become friends as did they.
In a perfect world, families would get along together, especially brothers and sisters, but that’s not always the case. Perhaps confinement with someone 24/7 for years and years takes a toll on sibling relationships.
Siblings know each other’s quirks.
Siblings know each other’s secrets.
Siblings know each other’s hypocrisy and lies.

Friends only know what is shown and revealed to them. Perhaps some childhood friends know a bit more.

I have no siblings, therefore, I have only friends. Many of my friends have been like brothers or sisters over the years.
I still have friends from high school that I keep in touch with. It’s like having a brother in a distant state that I never see any of them.
Those have stuck closer than brothers.