Mum is jealous of my mother in law

[warofthemonstertrucks]

My mum is very jealous of my mother in law. MIL is younger and her and FIL often come to stay with us when en route somewhere else. They are really helpful with the kids (two younger DSS's who are their biological grandchildren and nice to my two teen DD's). They are good house guests when they stay and fairly easy company.
My parents are older. They are frankly awful house guests. They don't come as much due to distance and immobility.
They have at times visited simultaneously. Everything has been fine when they are there all together but my mum has taken to slating my MIL later on (just to me and my DD's). It's really irritating. I love my parents but I also love my PIL and it's not a competition.
My mum seemingly has no awareness of how annoying she can be, and its getting harder to bite my tongue.
How do I get my mum to cut this out without being accused of favouring my MIL?
I've just had to listen to my mum slating her for half an hour over lunch and it's really not on.

Tell her a condensed version of what you said here? Just explain that you like your PIL and you don't want to hear anymore vitriol about them!

you could say that you do not feel comfortable talking about your MIL as she has been really good to you.

I would tell her to stop finding fault. It is though hard to communicate with a person that is jealous as whatever you say the jealousy will still be there regardless but that doesn’t mean you should have to listen to her moaning on. I’d tell her to change the record it’s very boring!