Of the immediate Simpson family, Lisa is the only one that has never been fully naked at any period of time. However, her scene near the end of the episode dressed in only a towel is the closest she has ever come to being completely naked. See more
Kids, your Great-Aunt Gladys has... passed on.
Gladys, Gladys... 'bout yea high, blue hair, big dent in her forehead?
No, honey, Gladys looked more like your Aunt Patty.
[thinks, then shudders
Oh yeah, there she is.