I see no problem with your mother not allowing your sister to have a friend over while they were gone. Your mother wasn't judging, because she has nothing to judge as she does not know this person.

She was just looking out for HER household as she is the one (or one of, since your father also has input.) that owns and sets the rules for it.

I can see your sister getting mad, as that's how most kids are if friends aren't allowed over.