the thing is these aren't "naturally" contentious subjects. It could be about something as simple as what color the wall was painted last year.
I feel ya. ToxicSis is like this. She'll pick a fight where there is none then berate you to death (I stopped having anything to do with her shortly before my wedding).
If the subjects aren't normally contentious, you could always try "Why are we arguing about this? This is a very silly thing to be getting angry about."
I tried this with ToxicSis; she would just continue with her argument (I've always felt her lack of something to do all day since she was/is unemployed contributed to this latest bout of crazy.
Also, apply some beandip. "It doesn't really matter what color it was last year since it's mauve now. Have you tried Aunt Carol's new beandip?"
One time when ToxicSis wouldn't let up (this was her berating me on my FB page), I successfully beandipped by posting, "La la lalalala la lalala la. Papa Smurf is my friend." Cracked up DH, anyway. Another time when I got really peeved (and, yay, led to her defriending me and finally leaving me alone once and for all), I posted a bunch of nonsense going off of lyrics from a song by the Dead Milkmen called "Punk Rock Girl."
My dad used to bait me all the time, too. It took a few times before I finally learned to give him as little info as possible. If just about all of your convos with CW start out with topics you are pretty sure are going to devolve into her obnoxiousness, I'd cut her off at the pass. Give her a silly answer -- "The moon is made of green cheese!" Give her a warning -- "I'm sorry, Dave. I can't help you with that." Look her in the eye and say, "If you continue to harass me, you will force me to report you to HR" or just hold up three fingers to her face and say "That's *three*" and walk away.