Is it possible that pre-sorting the laundry is enough trouble that if she has to do it for you, then she wouldn’t need you to fold it?
For example, if she has bad arthritis (I don’t know that that’s the case — it’s just an example), she may be unable to pre-sort the laundry for you without a great deal of difficulty.
For whatever it’s worth, I can guarantee you, there is no tznius violation if you fold her husband’s underwear for her as a chessed.
The Wolf