How to tell a family member I do not want any candid pictures taken?

My sister-in-law seems to think every picture she takes should be on Facebook. She even posts pictures of herself that are unflattering, her underwear or cellulite is showing (she wears a lot of short skirts), of family members while they’re eating, etc. I’m dreading the holiday visit. You wouldn’t believe some of the pictures she’s posted on Facebook!
I am trying to think of a way to request that she only takes posed pictures of me, and I will gladly stand behind her while she is snapping away with everyone else. Any suggestions?