I think you should have discussed inviting your husband's mom, but the deed has already been done.
However, I think your husband needs to reassess kicking his mom out of your lives because it is confusing to the kids & she hasn't been dependable keeping communications open.
From my own experience, I always think -- how will I feel if I were at this person's funeral? Then everything is put into perspective. Short of someone being a child molester, I always opt for contact or a relationship of sorts with immediate family members.
When people are hurt, they do hurtful acts.
Who knows what is really & truly going on in your mother-in-law's life?
Kindness & compassion goes a long way.
I am all for moving ahead & forgiving painful hurts of the past.
I have a dad who is very ill & I have very FEW good memories of him from my childhood. But I still try to call him weekly or every other week & have long conversations with him because once he's gone - I won't be able to hear his voice & just talk. I try to pick up subjects that he likes to discuss (his health; old family members; his past business career; the history channel). Our telephone relationship is the best connection I'm ever going to have with him before he passes on. So I'll try to keep up the every other week calls. I would never expect him to call me or remember important dates in my life. It isn't in him (or my mom either). But he has a lot of other good qualities that I focus on & try to forget the hurtful & bad parts of my memory.
My mother-in-law -- totally sane & sober -- told me three years ago right before the birth of our last baby ( I am married to her son right out of college) -- she told me she always disliked me -- bordering on hate & she is only in our lives for her son & grandchildren. My husband immediately said she had to move out of our home & he wanted nothing to do with her ever again. (His grandmother pretty much raised him; his mom always had to work.)
Eventually, after the shock, pain (and birth of my last child), I convinced my husband that it was unfair to his mom for him to completely cut her off. She hates me but probably because I married him (I'm really a likable person:) haha) So I arranged yearly visits and also the kids (and I) write long letters to her with pictures. I can't bring myself to have small talk on the phone with her, but I do have my husband call her monthly to check on her (she lives in another state next door to his sister now).
I think when my mother-in-law passes on, I won't have any regrets. I have kept the lines of communication open. I've inspired family trips to see her -- letters and so forth. My husband can't understand how I can be so forgiving. I just put myself in her shoes and think if I had the problems/issues she's had her whole life, I may behave the same exact way. "There, but for the Grace of God, go I." Plus, she gave birth to my husband -- my soul mate of my life. She can't be all bad:)
So I truly believe in connecting & being the "bigger" person so to say.
I have had friends lose both parents & even if the parents were psychologically abusive or alcoholics or whatever, they are still your parents. They tell me all the time - - think of yourself at your parent's or in-laws funeral -- would you have wished you behaved any differently when your mother-in-law was alive?
If you have worked through the sadness & pain of your
mother-in-law not living up to your expectations of what a mother-in-law should be, then the answer is probably -- YOU do everything reasonable to make her know that you care & she is loved -- even if you can't do it on a weekly basis.
I am sure there is a lot more going on than you could describe in your letter. But you did say "she doesn't call"; "she doesn't show up"; "she forgets the kids birthdays"....a lot of the relationship "up-keep" seems to be on her shoulders.
I would call her. I would go to visit her.
I would take her out to dinner. I would call her before the kids birthdays & say: "We just want to have you over for dinner.....but please -- no gifts -- not one little thing -- we are inundated with toys here. But it would mean so much to the kids if you came over for dinner. We'll pick you up at 4:30!" (sometimes for reasons unknown: moms/dads don't like to drive; or forget; or are scared of a scene -- arguing? or maybe even drink? so they don't show up when agreed & invited -- picking them up -- solves the "no show" problem)
(just side note: my brother was having tough financial times for a while & my sister kept fuming that he never remembered her kid's birthdays. I told her if you were having difficulty paying your rent & health insurance payments -- would you want to send even 20 bucks 3 times a year, plus Christmas gifts to your nieces & nephew? And would you want to tell anyone you were having financial issues? It just doesn't hit the radar screen when you have financial difficulties.
So I never think twice if presents don't materialize. But I do call my parents on the day of my children's birthday & then, I call my daughter or son to the phone & say "Grandma is calling." The kids don't know who called whom & sometimes it takes a lot of calls to get my parents to pick up the phone.
But the end result? My children are thrilled & feel connected to my parents -- and know that more people love them.
And in their hearts, I know my parents love my children more than anything -- they just have their own difficulties & can't see past them sometimes.
The long & the short of it:
Always try to be as compassionate & connected as you can with family! (and try to persuade your husband to be too -- the funeral idea usually works for my husband:)