I like FB, but I unfriend or don't accept requests from people who I don't want to know my business. I also only comment on things that interest me.
I say get rid of your SIL or anyone else who you don't like. Buy the way she sounds awful and it would upset me too to see someone putting down their own kids on a public forum. Good for you for calling her on it! But try not to let it get to you too much. You can't control other people.
Never argue with an idiot.... they'll only bring you down to their level and beat you with experience.