Her parents let her talk to them/everyone else however she wants to. Today, I was in a crabby mood, and I was volunteered to pick her up and take her somewhere (still not sure how that happened... haha). She was being so sassy to me and I snapped back. Everytime I'm around her, she acts like everything has to go her way or else she throws a hissy fit.
It makes me worried about having children and what they will turn out like... DH assures me we won't let that happen... but it still scares me.
Re: I had to deal with my pre-teen SIL
Kids need limits. As a teacher, even though a child might be sassy with other adults in their lives, I never let them be sassy with me. If they try to be, it's not pretty. Kids do need to learn to respect others, but the respect only comes when you demand it (and of course, when you respect them. Being strict and being condescending/insulting are two different things.) I would tell your SIL, "Hey, you might talk to other people like that, but you are not allowed to talk to me like that!" Her attitude will probably shift. Most kids don't even realize that they are being sassy in the first place.