What do i do when my brother likes my best friend?

I saw no problems at first but when they hung out more it just seemed to get worse. They are always like treating my like **** in front of each other. Whenever shes over, she always asks me wheres your brother? and he constantly has to be with us when we hang out. Im fine with them separate, but it never seems to be that way anymore. And i found out that they text a lot too. Its just not the same anymore.