The reason why your MIl is nice to your DHs ex is she loves her grandchild. She may not care two pins for his ex wife, or not. Her own children and grandchildren are blood relatives, partners are not ( they can also come and go)
However, as a parent and grandparent with grown up children, When they were going through the dating stage my Dh and I met loads of prospective partners.
We grew to like and love some of them, especially if they were around for a couple of years.
If the relationship foundered, these people were toast. Despite welcoming them into our house, entertaining them, sometimes paying deposits for apartments, cars etc to help our children. Once they were gone, we were not allowed to have a relationship any more or MENTION them. It is quite hard.
Luckily, the situation has never been compounded by children.
But if my DCs had a child and they subsquently divorced, I would want to keep on good terms with the ex. Not because I necessarily liked her but because I would love my grandchild.
I don't condone your MIL ,not being straight forward and honest, as we all should be, but she has a lot of balls in the air trying to keep everyone happy.
She has your DPs ex, which is difficult, she has you, which is difficult, and she has her son.
Mmm cut her some slack and calm down.
Your attempts now to convince us that you are a reasonable person without a myriad of your own issues, I am afraid falls down, because your language gives you away.
You also say you are not honest and straightforward either as you have been unable to say anything to your MIL about her behaviour. You want her to be honest with you, but you are unable to be with her. You just seeth about her in private.
Your DSS also might be well aware that if he mentions the Ex ( his DM) he will get into trouble with you. Children in this situation learn to lie quickly.
Your MIL will probably sense that you are annoyed anyway and your DP will be telling his mum what you are annoyed about.
These people are going to be in your life for a long time. It might be that your MIL is a Narcissist, but the only person you can change or do anything about is yourself.
You appear so angry and upset, so perhaps some counselling and a mindful course might help you.
With all the exs, and the step , this and that, it appears to be a relationship mine field.
Do you want your DP to give up seeing his MUM?
I feel sorry for you OP as you sound so miserable. Instead of seething, be honest with your MIL in a calm and adult manner. Tell her what is upsetting you.
Make sure you can answer the question ' Am I jealous of my DHs mother and his Ex wife, with a resounding ' no'