It's from the episode where the family goes on vacation to the Flanders' lake house in that town with the absurdly long name that I don't remember and Lisa starts dressing differently and makes a group of new friends, one of whom is Christina Ricci. I think it was the seventh season, I don't recall the title.
EDIT: Sorry I might not have followed the rules here correctly, I am stoned. The joke is that Marge says "I'm never gonna let you go" or something like that to Maggie because Lisa is trying to be independent or something, and Maggie is worried because her mom is being clingy.
EDIT 2: Sorry again, I actually took a look at this, uh, sublemmy's other posts and it appears that I was actually supposed to respond with a quote from the episode. So like, you know, whatever.