My mother-in-law made it so easy for me. She started introducing me to family and friends as her daughter. Not daughter-in-law mind you, but her daughter. So it was pretty simple to start calling her mom 
I have to say one thing, she is a wonderful woman whom I love dearly. She did a pretty damn good job in raising her son.
I guess it depends on the relationship. It sounds like you started out kind of rocky, and those things are hard to get past. I called my late FIL Dad, to differentiate from my own father, who is Daddy. I don’t speak to my mother, so Mom or Mum was okay for my MIL.
It is a little odd after four years to suddenly ask her what she would like to be called. At this point, I would say call her by her first name. It’s most familiar to her, and it would probably sail right by her that that’s how you are referring to her. It’s the first time that’s the hardest.
Between my wife and I and all the in-laws, we all just call each other by our first names and it works very well. All the relationships are very (extremely) loving and cordial, with the possible exception of my MIL and I, which is probably better described as guardedly respectful with possible modifications due to her neurosis du jour.
She is a neurotic perfectionist, does everything by the book and can’t comprehend that what works for her might not work for every other member of a diverse demographic. Accordingly, while every other grandparent enjoys a cutsie, lovable nickname, she’s just Grandma.
The only time I didn’t call her by her first name, it was a mistake. On the phone her husband sometimes refers to her as “The Big Woman”, an endearing term means that she calls the shots regarding the household, nothing more. I misremembered the term " The Big Woman" and during a conversation once kept referring to her as “Big Momma”. Well, she’s active in the church, has won Woman of the Year a couple of times, is on the bank board, the school board, President of the Theatre, etc, etc and apparently took offense at me referring to her saying “is the Big Momma gonna do this?” and “is the Big Momma gonna do that?” She got over it in due time though… about 7 years.
My brother-in-law as you beat – in 35 years, he has never directly addressed my mum and dad. Mum knows, and has no problem with it, as she’s very understanding; and she and Tommy are actually very close.
So no worries!
I was with my first husband for 10 years, and never once addressed her by any name or title. It was the same deal; I wasn’t comfortable calling her Mom (or any derivative thereof), couldn’t bring myself to call her by her first name (or last name, for that matter) or anything else (including the Millie/Phil deal). When I had kids, I had something a reprieve, I could refer to her AS grandma (go bring that to Grandma, where did Grandma go?, that kind of thing) but couldn’t actually address her that way. I never did figure out what my stumbling block there was - she was a perfectly nice person.
My current mother-in-law? I call her Ma. So go figure.
When Eric and I started going out (six years ago), I called his parents Mr. Lastname & Mrs. Lastname. That lasted less than six months. They insisted that I call them by thier first names. The Mr/Mrs thing “made them feel old”. So when we got married last year there was no question as what to call them.
He addresses my parents by thier first names and I address them as mom & dad, of course.
I address his grandmother as Grandmere Mary (as I was instructed by her) same as Eric.
When Eric and I talk to each other regarding our parents, we just refer to them as “The Ps”.
Even though I now have Eric’s last name, I feel weird being called Mrs. Lastname (That is his mom, I am Becki Lastname). When that happens I still look to see if Eric’s mom is around :o 
I had the same dilemma.
Calling them Mom and Dad was out.
Calling them by their first name wasn’t done.
So I didn’t call them anything.
Until we had kids.
Now we call them Oma and Opa. (German for Grandma and Grandpa.)
My mother refers to herself as The Big Bad Mother In Law to my husband. But he doesn’t call her anything either other than Trouble.
When my kids marry I will make a point to tell their spouses that they can call me by my first name. Just to make things easier in the long run.
<George Costanza>
I would finally have the chance to say I wanted to be called T-Bone!"
</GC>
I’m so glad I’m not the only one with this problem. My MIL and FIL want to be called by their first names, and the other 2 women who married their sons have no problem with it. But for some reason, it’s always made me uncomfortable.
I remember my mom’s negative reaction when one of her DILs called her “mom”. It must’ve scared the other one, because she didn’t call her anything. But my mom was a bitch, it didn’t take much to set her off. My MIL is nice, but I still can’t do it.
My first wife’s parents introduced themselves as <First Name> and <First Name> and that’s what I called them.
My present wife’s parents are both dead, so it isn’t an issue. She doesn’t know what to call my parents, either, so this must be a wide spread problem. My SIL calls them “Grandma” and “Bobo” (which is what the grandkids call them).