This is so eery, this is what happened to me (although a lot worse for my mother) and was  the thing that finally ended my toxic marriage. I still struggle with it, thankfully I believed my mother as I had seen and/or experienced my husband's strange behaviours in a number of different contexts by the time he assaulted her.

I have experienced this in my last 2 positions (totalling about 10 years). I'm in the process of extracting myself from my current workplace and have decided that there is ironically more psychological safety for me as a contractor where I can change jobs on a whim than in permanent roles where I feel I owe something to my employer. 

So my advice... Block her. There is no reason for you to be communicating with her. Everything goes pear shaped as soon as the girlfriend/new wife gets involved. I speak from experience as first the girlfriend who got too involved (sorry!) and then the ex-wife. The male is at fault yes but she should respect the boundaries. You need to make yours explicit. 

Why did she contest the Will if both sisters were left equal shares of the estate? 

I really don't like people referring to my kids sisters as half sisters though. You can't split a person in half and to them they are their real sisters regardless of who their parents are. This was particularly a big deal for my kids when they were little as others felt they had to define and label their relationships with the bigger girls. I think people need to just accept that families come in all shapes and sizes. Definitions don't help.