If something is important and you don’t have it, don’t lament. Replace!
Siblings are an incredible boon; if you only have one child, it’s great to give them siblings. But if you can’t (or don’t want to for other reasons), find surrogate siblings! Find the kid they like best in the daycare or school and become absolute best friends with their parents.
This mentality works for all sorts of things. People can be important figures in your or your family’s life without having been born to the title. Don’t chase them away just because they aren’t the “genuine article.” There’s no such thing, only those who genuinely bring joy to your life.