I think it's fine UNLESS they are banding together in joint hostility towards you which is not, obviously, but even so there's not a lot you can do about it.  If you have a look at my most recent blog, you will see that I had a bit of a similar problem, some time ago NPD BM and MIL became Facebook friends after DH and I became estranged from MIL due to her awful behaviour to us on a visit there.  

The BM is visiting ILs every day, to see her kid (why not living with him, this is another question). She drinks coffee with them, smokes her cigarets and often has a diner. She even has her name on their postbox, ILs are helping her to avoid going to the post office and collect her mail. They also help her financially. In fact, in addition to my DH paying her CS while the boy is living with the ILs, they buy furniture for her house, clothes and food for the boy, even when he stays at his mother’s. 

SIL and BIL and their partners are also friends on Facebook with her, they like her pictures, share her posts and so on.

I simply reduced my interactions with all of them almost to zero. Unfollowed them on Facebook and stopped texting ILs as I used to do before. I am sick of them saying bad things about  BM behind her back and being BF on social media. I am sure they are saying bad things about me to the BM too, so I do not news feed them.  

You need to let this go.
It doesn't matter that BM and FMIL are friends - they were friends before you were in the picture and your SO divorcing his wife doesn't mean they can no longer be friends.
I don't understand why you want to police the relationships of others or pass judgment on it: you don't have the right too.

Concentrate on what you can control.
If the relationship makes you uncomfortable, limit time or exposure to FMIL and BM.
You - or SO - don't get to control who is friends with whom because your SO has a past divorce.

I don't think they care how you feel about their relationship - why should they? -  and your dwelling on it only gives them unnecessary time and space in your head.
Let it go because it is not worth your time or aggravation.

I think it’s weird and I would never be besties with my ex MIL, even if they were grandparents of my kid.  If BM is doing it to keep a foot in the door in DH’s life, or if the ILs freeze you out in favor of BM, then it would be an issue for me and I’d have little to do with the ILs and my DH would do the same. 

It’s true you can’t control their relationship but you don’t have to be mistreated and put up with it either. 

If MIL and BM have a healthy relationship and it in no way involves you and your DH, there isn't an issue. However, once they start chatting about you two, or MIL still refers to her as "the wife", or oversteps boundaries in any other way, it becomes a problem.

My MIL and BM are besties here, and DH and I both have little to do with MIL because of it. MIL would rather have unfettered access to her grandkids than show respect to her own son. Basically, since DH isn't CP, MIL feels she MUST befriend BM in order to see the kids. She doesn't like that DH would limit her time with them to a weekend a month, since he only sees them EOWE and has other family and obligations with them.

DH has no real relationship with his mother because of that and the fact that she and BM have the same personality. Both suck at money management and expect others to pull them out of their own messes. Both cheated on their husbands. Both are slovenly. DH, FIL, and I all joke now that DH married his father when he married me, and yes, that is a fabulous compliment. 

DH can't control MIL's relationship with BM, so he controls his relationship with her. He has now gotten to the point where, since she is buddy-buddy again with BM, that she can go to BM to see his kids. When BM and MIL have another falling out (and they do), she's up sh*t creek in regards to the boys. DH won't facilitate that relationship outside holidays and their birthdays. It's the consequence of getting in bed with the enemy and helping her make DH's life hell.

I can see how this would make you feel icky or uncomfortable, but you've mentioned that you & MIL get along great, so I'm assuming their relationship hasn't impacted MIL's relationship with you or her son.

You can't control who other people have relationships with. They might just genuinely see one another as friends & there's nothing wrong with that.  If MIL was meddling in your FH's relationship with his kid, or taking sides in or causing drama between BM & your FH, that would be an issue.  If that's not the case, however, as weird as it may be you're going to have to find a way to make peace with it.

I think it’s weird and usually a control tactic by BM to stay relevant in her exes life and hold onto her power.

my DHs tried her best to remain bffs with MIL and FIL and his wife....all it did was cause turmoil. For example....BM came and snatched the kids one Christmas because they had “plans” the next day we found out that the plans were that BM took the skids to FILs house to spend Christmas.....BM didn’t tell DH and FIL assumed DH knew....that was the end of that. 

Can you imagine your ex coming to take your kids from you on a holiday just to got over to YOUR families house without your knowledge? Crazy 

I think children are often used as an excuse to not move on from relationships. Would your MIL be friends with BM if she and your DH were married and divorced, but did not have children? Seems like the BM is using her “power” of having children to continue to keep ties with DH’s family.

If MIL talks at all about your DH or you with BM that’s a big problem. She needs to respect your boundaries and not share those things with BM.

Overall, I think it’s weird and can cause unnecessary drama and heartache, especially if your MIL is the meddling kind.

My bride and my mom are BFFs. I never win an arguement.  *wink*

If this marriage were to end in a break up... I would never be rid of this relationship.

Fortuneately, things are looking good so far.  She has put up with me for nearly 25 years.

When you say besties. Is BM using it to spy? MIL inviting BM to family events?  MIL still hanging the old "family" pictures in the household?

I don't see the issue with them being friends as long as it's not just to snoop and she's not at family events or being weird about it, because THAT seems inappropriate to me.  But if it's totally seperate, then it's her choice.  

Would it still make me uncomfortable? DEFINITELY.  But I think I could live with it if it wasn't weird stuff like that.

I will however NEVER understand why MIl would want to be firend with her son's ex, or why BM would want to be friends with her ex-MIL. Just seems a bit weirdly enmeshed to me.

LOL got the same situation going on lady! My SO and BM were barely together 3 years  and she lives directly behind my SO's parents. She is there almost every day, attends holidays, birthday parties, celebrations and family day on the weekends. We have disengaged. I have no contact with BM or MIL anymore. SO has little to no contact unless pick up/drop off.

We both had the understanding that there would be a relationship because of SD. Not full blown enmeshment and moving into MIL's backyard....literally.

But disengaging from the toxicity of it all has done wonders.... You cant make this shit up!

OP--I have an adult who is married. Also I am a granny.

There is NO way in hell IF my bio child divorced would I be bff with my former kid-inlaw out of respect for my adult childs decision to end the marriage. I would be polite in all social settings because it is the right thing to do.

In my opinion your MIL has crossed a line of family loyalty.  Sounds like MIL Needs a to take up a hobby and have BFF's of her own age.

Very awkward of MIL...

The problem is not that BM and MIL are friendly, the problem is MIL has repeatedly breached your trust and repeated private personal information to someone who used it against you. She has no loyalty to your DH. I'd go no contact with MIL.

When my brother divorced my parents and I both remained close to his ex wife.  She was the mother of their only grandchild and all lived about 1200 miles away. She is the only one who made arrangements for my parents to see their GS.   She even paid for their airfare to come see him. 

BUT no one ever mentioned what my brother was doing with his life nor was the ex ever invited to family events. There was an appropriate separation of interactions with both. 

If MIL can't learn then she will have to feel the isiolation of not hearing one word about or participating in the life you share with her son.  I have had to give my own mother clarity on this a couple of times over the years.

The primary issue was her sharing with her BFFs details of my life that should have obviously been confidential between she and I.  I had to be very pointed and assertive with her to let her know that if she could not maintain boundaries of confidentiality regarding our conversations.

I would suggest that you and DH make sure that you are aligned then implement a isolate MIL from anything other than insignificant trivial information regarding your relationship and the family that you and SO are making.  MIL will pick up in short order that she is being distanced from her son's live.

Good luck.

If it's healthy and she's a normal human being, I think it's alright, but I don't think paths need to be crossed.  I have a decent relationship with my Ex's dad.  But I'm not a psycho, lol.  On the other hand, BM in my situation is a mess and MIL is an enabler.  For 2 years during the beginning of my relationship with DH, MIL invited BM to Christmas dinner.  The 2nd one was our first christmas living together and DH picked up BM from her house and drove her to MIL's while I spent Xmas eve home with my 2 boys and our brand new puppy we got that afternoon!    (Oh man, I have had a lot of therapy and plenty of fights about THAT one.  Not sure how he's not dead.). BM has also moved in with MIL TWICE, the most recent being last fall.  I ripped into DH and told him that he needs ot tell MIL that we will NOT be attending Xmas or any events at her house as long as BM is in the picture and there will be no relationship with anyone in our household.  DH made her cry and felt awful about it, but it's what she needed to hear.  BM moved out a month later.  SHe's a low life loser on welfare and sleeping with whomever to get her bills paid.  She actually ended up moving into the cottage of the man who lived next to MIL, LMAO.  He's also MIL's age.  BARF. 

God, I wish she was a normal human being.

Honestly I don't see a big problem about it.  Unless they're being actually disrespectful to you, it's all the better the better everyone can get along.

I know first hand exactly what you are feeling. My entire husband's family is VERY friendly with his ex wife. It bothers me bc they tell her everything that goes on in his family! It makes me extremely uncomfortable & it causes issues with me being able to trust his family. It also bugs me bc no one has asked my husband how he feels about the things that they do, like inviting ex to family functions, inviting her to walk in the funeral with the entire family & sit with the family, her showing up at every family emergency, I could go on & on. To me they are showing her more respect than they are there own son, nephew, cousin, etc. he divorced her for a reason & doesn't want to deal with HIS family inviting his ex do all these things. He has spoken up a few times but his family still does what they want to do, which is why I don't have a relationship with them. He did tell them eventually they would have to choose her or him!