My SIL is a stepdaughter. Her mother passed away when she was very young and her father (now deceased) remarried Stepmonster not long after. SIL has 3 step-siblings. SIL and my brother have been married for 40 years and have 2 children (adults). Their kids are the only ones who have ever met the step-siblings. I couldn't even tell you their names.

SIL is damn good to her SM. So much so, that SIL is the first person that SM calls when she needs something - which is often. SM's biological children are always "too busy" to do anything for their mother. Doctor appointment? SIL takes her. Grocery shopping? SIL takes her. Ceiling fan needs dusting? SIL does it.

Thanksgiving was at my brother's home. Naturally, Stepmonster was invited. An 85yo woman with a high school mean mentality. When her husband was alive, she treated him like crap - he couldn't do anything right, talked down to him. Every time I see that woman, she is catty and hateful.

Three years ago, one of my nephew's and his wife had a new daughter. Stepmonster didn't "approve of" their baby name.
SM to me: Sorcha. How awful. Don't you think that's an ugly name? (Mommy is Scottish.)
Me: I think it's a lovely name.
SM: You DO?!?!
Me: Yes. I do.
At which point, my sister chimed in and said, "I think so, too. Unique and pretty."
Stepmonster looked at us like we each had 2 heads, shook hers, and mumbled to herself.

So, Thanksgiving. DH and I were sitting on the couch. He worked an extra-long shift the night before and was tired and sleepy. I fixed a plate of appetizers for DH and one for me, handed him his, sat down and picked up my wine glass. Stepmonster says, "Wow. You baby your husband." I gave her a bored look and said, "No. I simply don't mind doing little things for him since he works so hard."
IMO, Strike One.

About 20 minutes later, I took our empty plates out to the kitchen, refilled my wine glass, and got DH another beer. Returned in time to hear Stepmonster say to my sister (the only other person in the room at the time), "As soon as Aniki sits down, let's get her to take OUR plates to the kitchen and get us more drinks." My sister said, "I'm not doing that." and turned back to watch the football game.
Strike Two.

SM: Aniki, have you lost weight?
Me: Yes, a little.
SM: Well, good. You need to. You're pretty damn fat.

What.The.F*ck. 
What kind of person says that? Yes, I'm overweight, but I'm not a freaking beached whale. And Stepmonster, who is built like a stack of tires, has NO room to talk. That was the last bloody straw and my temper - usually a unicorn - let loose.

Me: I feel sorry for you, Stepmonster.
SM: WHAT???
Me: I feel sorry for you.
SM: SORRY for me - why??
Me: For whatever happened in your life to make you so negative and hateful that you find it necessary to talk down to people and treat them like shit. You must feel terribly ugly on the inside to try and make others feel badly about themselves. I know you're old, but it's not too late to go to therapy, get help, and strive to become a decent person.

Stepmonster's mouth was flapping like a fish out of water. My sister, brother, nephew, and nephew's wife all started laughing. I was mean to a mean old bat. Going to he!!.