[Overpaid Slacker]

I agree with most of what's been said here ... the be "firm" and "resolute" parts, most especially.

I'd add to that recipe "forthright". It's obvious this is beyond the point where things "will become" a problem -- they are a problem. And it appears that the basic options are -- you and your wife can take action now or be forever upset with your sister (and maybe w/ yourselves for not having taken any actin).

You risk that the sister will be upset with you, but you owe her nothing and have been more than tolerant and generous to date. When and if she ever grows up, however, she'll understand that the world (and everyone in it) does not owe her anything and acknowledge your generosity up to the point where she'd exhausted it.

I think Red-Beard's "leave her a bill" idea is a bit extreme, but if that's how close you are to your wits' end, perhaps you should. In any event, you -- and your wife -- need to have a verbal intervention and lay things out. Do not allow sis to cause you anxiety in your own home; to dread "rent time" or to let things slide to your detriment. There are actual, marginal out-of-pocket costs to her living with you that she is inflicting on you, and evidently doesn't care about so treating you and your family.

Whatever you decide, be forthright, do not allow her to evade/elude with gossamer promises, and, when you're done, look her square in the eye and say "you're not going to 'forget' this conversation and the arrangements we've agreed to, right?"

JP