My mother in law is wonderful. I love her so much. You could just sink into those arms and stay in them forever. I honestly have never in my life met anyone with a presence that is more comforting.
I remember when she came over to stay, when my first child was a few months old. I was really stressed one time because I couldn't get him to take his milk. Me and my mother in law don't speak the same language but she clearly got what was going on because she said something and held out her arms and sort of beckoned. I practically chucked him at her! 
She said something to him, tipped him a bit and he drank the lot, no problem and fell asleep 
She has no idea how close I came to burning her passport and forcing her to stay forever 
My own mother, otoh, is and always has been a bit crap
bless her. If she was my mother in law, she'd be a classic one 
I think people generally are nicer/more forgiving to their own mothers. Behaviour that would earn your mother in law the label 'toxic' is justified, excused, accepted or just plain not noticed if it's your mother.
And what is loving involvement from your mother - eg phoning you loads, coming round, wanting to be a big part of your life - is seen as overwhelming, interfering... from your mother in law.
A comment from your mother is helpful
The same comment from your mother in law is critical
Your mother coming over every day is fine. She's your mum!
Your mother in law coming over every day is Just Too Much...
Some mother in laws never let go of their sons and think they should come first
Some daughter in laws think that their own mother is more family to their own family unit than the mother of their husband is.
And then there are the actual horrible people. On either side. The control freaks. The ones who believe they're still in charge. Those who don't recognise their offspring as independent adults.
Then you've got the out and out bonkers.