A valued, trusted employee. Not part of the family. Not really a friend either--would you move into her house for three months and take care of her kids and her sick spouse if she needed it?
(Who's taking care of her kids during that period? Or are they moving in too?)
I just say that because it's significant here; even if she does stay the whole time, she's not an equal family member, but an employee. You can tell her that she'll have no negative consequences for "telling on" your wife and she'll pretend to believe you, but we both know that's not the case. And your alcoholic wife is going to exploit that power difference.