Absolutely tell her! With your other comment about them staying over night regularly, you also want to be on the look out for them taking matters in to their own hands.

Also, if you’re looking at popping the question, some advice about being married: it’s the two of you vs the problem. Not you vs her vs the problem. Tell her about this so the two of you can move together jointly. It doesn’t have to cause a rift if you tackle this as a team.