Welcome to Psychcentral.

Why are you allowing your SIL to use you as a verbal punching bag? I used to get really upset about certain things my husband did/said, now when he is being poopy to me I ignore it. I know confronting it will just start a fight. I grit my teeth and clench my fists often. You need to talk yourself out of the confrontation, it just is not worth it.

Also, this is so contradictory; "I usually end up laughing at her and walk away, but now she has so much hatred and animosity I don't know what to do for the greater good of the entire family. I've tried for years to keep the peace, play nice and be the diffuser (sic) of her wrath."

Which is it? Do you antagonize her or try to keep the peace? From what I can tell you are not keeping the peace or diffusing anything.

I am sorry she treats you that way though. Being bipolar is not an excuse. Does she live with you are something? I do not understand how she is such a problem for you that you cannot just avoid her antics.

Also IS your SIL bipolar or did YOU diagnose her? It really grinds my gears when people use my illness as a way to cut someone down.