in streaking it also seemed to imply that donna was an only child. and that was earlier than eric's burger job. maybe she wasn't really donna's sister and was actually donna's little sister for a school project like that time jackie had a little sister lol. 220.127.116.11 16:33, January 17, 2011 (UTC)
I'm pretty sure she was mentioned in another episode after Midge came back into the series after being away for a bit. Something along the lines of "You don't want to end up like your sister."
Just don't remember the exact episode.