ymartinez,
Your mother-in-law’s being “great friends!” with the ex’s mother is strange indeed! How awkward.

I would think the ex-'s mother would now be busy with friendships connected to her daughter’s new life, not her old one.
I can certainly see how this is very awkward for you. Yes, making overtures of friendliness with your new mother-in-law is right. See how she returns these (its a give-and-take relationship!). But trust is earned. She has a strong loyalty to the mother of your husband’s ex, so its reasonable to be wary.
You don’t need to be her best friend, or even be a friend at all. Friendship, when possible, takes time to grow. And it takes two! You owe her polite respect*; she is an important person to your husband. Closeness is earned, though.
*[Polite respect: like you would be professionally, to people at work. Polite inquiries about her health or hobbies, or the recent weather or her garden’s progress. Even polite respectful benign inquiries about the health of her great friend! (acknowledging thereby that you see where her loyalties lie).]
If your husband’s relationship with this woman makes you uncomfortable, he should know that. You can make it clear, brief, and non-emotional when you say so, and only repeat it if necessary (like, if after time, you do not see evidence he is taking that concern seriously). Give him time to do what may be hard for him, and if he does not respond right away (perhaps because he is hoping it really doesn’t matter too much to you, so he doesn’t have to make any uncomfortable changes), you can politely say so again, calmly, clearly - as if you had not mentioned it already before! Again give him time to respond. (This is a teaching technique called “the broken record”. There is no emotional: “Why didn’t you do it when I asked before!”, but just a calm repeat of the original request).
Everyone’s allegiance
should be to
you two. Hanging on to the remnants of the couple-that-was has a certain disloyalty to it. And your sensitivities should be considered. If they are not, well, then you have no need to be any friendlier than you feel like. I say you can be cool and polite until you feel safe to be more.
In my previous marraige my ex’s family was dysfunctional in various ways, due, for one, to the parents of this large family who had some substance abuse issues, so all the kids grew up with various issues.

There was good in the family, but plenty of dysfunction, too. Anyway, I was always making an effort to be friends/friendly with this family** - I truly gave all, and overlooked all slights, but friendly overtures were never returned. I just wasn’t important; I just didn’t matter, because* I did not grow up with them* (so I was not a
real family member. Even though I took their name, I was not one of the amazing Smiths!) I got only
more disrespect for my effort.
A brother-in-law’s wife took a different approach, influenced by how she saw me act and how they treated me. She was cool, polite, and ANY disrespect shown her by them got addressed IMMEDIATELY, and STRONGLY. No one had
any doubt she was offended, and why! She did not care one whit that it might seem petty to anyone or everyone - to her it was not! And truly they
needed this, because all the sibs affirmed each other that they could all do no wrong.
So she earned a certain respect. That I never got. She never was their doormat. Like I was. On top of that, this was truly better for her kids.
Well, everyone is different and this was not me. But clearly I could see, in retrospect, that her way worked better than mine. I should also explain that her husband was oblivious to the disrespect turned on her. But she did not dwell on his failing in this area, but just accepted this reality. Since his support was not there for her, she was assertive about expecting people to respect her ON HER OWN. For example, if an in-law was rude to her when she and her children were visiting them in their home, then she (
and her children) did NOT turn up at the next family function, *or *the next - for some time, if at all. She would wait for the apology, which this family was scant of. Her husband did not like his wife’s boundaries with his family, because, like my ex, because of the family dysfunction, they had
extreme loyalty to their family of origin and could not admit any one of their sibs had made any errors. (Truly they married and did not cleave to their wives but to their sibs!).
I have to say, all the other brothers in this family did not keep their wives and families. Only this one. I think being the wise one who stuck up for her own and her children’s boundaries is a big part of this.
Well, hopefully you won’t run into any of these problems, but just remember, sometimes you cannot depend on others to defend your boundaries for you. Its nice if your husband does, but not every man is able to do that. Its hard to go against the set-in ways of family-of-origins. So some communication may be in order, and you can be clear and calm when explaining yourself. But if in the end you do not have his support, its up to you to make your own boundaries.
** I thought trying to be “family” to my in-laws was best/right. But later I read the advice of Miss Manners on how to behave with in-laws. She said you should
play the part of an unobtrusive anthropologist, observing a
strange culture!
And actually, this would have probably gone over* better *with this family, than trying to be one of them. Because they didn’t
want anyone else to be one of them.

Anyway, Miss Manner’s advice is very wise. You have to accept what you have, and work with it. (And we are all dysfunctional in some way or another!

)