Well, the thing there is that if it’s your sibling, you absolutely have to take his side. I was good friends with my brother’s ex-wife but once the marriage ended, and not amicably, that friendship had to end. Now, in Ross and Monica’s case, as long as Ross doesn’t end the friendship, Monica is fine with continuing it as well. But if Ross had actually gone through with cutting Rachel out of his life, then Monica really should ask Rachel to move out, because her brother should trump any friendship with anyone.