You kinda do that with everybody.  Disengage with BM is OK.  I could not be friends with my SO's Ex.  Just remember, you let your SO do the picks up and drop offs.  No joint birthday, party, joint hollidays, ect  No going in on joint gifts for SK. 
BM is your SO Ex. That's all, when they divorced there relationship ended,   Old lovers can not be friends .  So no doing anything for the ex

I haven't seen BM in years, except for at court, but I ignored her as well, once it became clear that her intent was to alienate SS from DH.

When they were trying to co-parent, I would say hello, but that was it. There is no need for an SM to be involved with a difficult BM. If BM is sane, then fine, but if she's toxic, trying to be her "pal" is just playing the game.

I tried being nice and doing the "we're all in this together" thing, and it bit me. BM's shenanigans affect my home and life minimally when I don't interact with her (because I have a soft heart and don't want to be mean to the boys' mother) and she has stiff boundaries.

I think you're figuring it out just fine. I came from a blended family, but that doesn't mean it prepared me for this. It means I tolerate it better and understand some of the reasoning behind decisions. But I made the same foolish mistakes that many new SMs who think they are doing or want to do the right thing do.

We live very close to BM, literally a few streets further. 

We very rarely bump into each other, when we do and my step son is present everyone says hello to step son but not really to each other unless BM and my DH have something they want to say. 

I've bumped into BM out and about with my step son and I wave and say hi, dont hang around but dont ignore. If me and BM see one another in the street and step son is not present we just ignore. 

There was ONCE that BM needed to drop step son with me, it was a one off since step son was ill, BM had misread DHs email and needed to work. I was available so I took him, I handed her some odd socks and said I wasn't sure if the pairs were at hers as step son prefers wearing odd socks. That was the extent of our conversation.

I love it lol 

I made the mistake of taking the bait when BM reached out with an 'olive branch' a couple years into our marriage. I have regretted it ever since. I wouldn't p*ss on her if she were on fire but I pretend to be her 'friend' for the sake of keeping the peace. It makes me feel two-faced...something I never want to be. If I could go back in time, I'd block her d*mn number and her profile on FB. Counting the days until both skids are 18 and I can finally let the truth come out.

No contact with BM will make your life so much easier. Your H is doing his part by co-parenting and you get to keep your sanity. It's pretty much a win win to me

BM and I more or less ignore each other.  I started out acknowledging her and generally being pleasant.  She decided she'd rather be a miserable something and so I let her.  That's her boundary, so be it.  

The few times she can bring herself to speak in my general direction it appears to be damn near impossible for her to be a decent human being.  I'm pretty sure she's not very nice to waitstaff or anyone in the customer service industry.  That's just the vibe she's got going.  I don't know that I'm special enough for it to be just for little me but I do think she turns it up a few notches when it comes to SO or I.

I started off disengaged so we've not had a need to clarify who is THE MOTHER or the like, she's got no real beef with me.

At this point she couldn't change the dynamic if life itself depended on it, I've no interest in participating.

BM here is sane and mostly reasonable, and I do have contact with her.  We text about pickups/dropoffs (I'm usually the one doing them since they mostly happen when DH is at work) and we'll chat if we run into each other.  I've drawn the line at being her friend on social media.  We used to be social media friends, but I decided I didn't need her knowing my business and I unfriended/deleted her.  I'm still friends with some of her family members, so she can probably see my stuff if she wants to, but I feel better without her having direct access.  I do not feel like BM is a friend, and I don't particularly like her, but we get along and have a civil relationship.

We live in a small town and I knew Meth Mouth from school and a previous job we both worked at. She was the snotty b*tch who acted like she was super sweet, but was crazy AF!

When I started dating DH and found out who is ex was I knew I wasn't going to have anything to do with her. She didn't remember me because after all I didn't run in her self deluded circle of waste, but she sure got a slap in the face the first time she tried to get me in line with her bullsh*t and d*ck yanking.

After that she just did everything she could to smear me to Spawn and anybody who would listen. Oh well that all blew up in her face after she started getting arrested and details of her gang banging and drug use started getting around town, and before you know it she's just another useless tweaker in town with no credibility.

My SO told me not to be friends with the BM because they're not even friends. BM is bat shit crazy!!! I've heard her scream and yell at my SO about some petty shit. It scared the hell out of me. So, I made it a point not to be in any way in contact with her although she had asked my SO for my number. Like, wtf why? BM always make things awkward for all of us. She would call my SO at 10PM for something that is not important. She will use her kid so she can have join in the convo with them. The thing that pisses me off is BM will bring back their memories out of nowhere and she knew I'm with my SO every time she does that. *eye roll* She's pathetic and always want my SO's attention any way she can get them. She even said a few months ago that she was dying. But she wasn't, of course. 

Then one time, BM asked my SO why I'm not friendly to her and why I ignore her. LOL. SO said, it's better that way. Haha! 

I never imagined that I would be friends with BM - she and I could not have less in common and even without her histrionics toward DH, I find her to be juvenile, vapid, and the version of basic that is not cool enough to actually be basic - however, I did assume we would be cordial to one another. 

Oh how I was wrong. After one brief meeting, where she reprimanded my DH (who was my boyfriend at the time) and barely acknowledged me, she sent a FB friend request. I didn't accept it and she lost it. She has cried to DH about this multiple times. The irony is that four days after she sent it, she withdrew it, so even if I had wanted to - which I didn't - I could not have gone back later to accept it. She expected me to accept it immediately.

After DH and I got engaged, she decided she would try to trash me. She told all of her family and DH's family, via email, that *I* wanted DH to move next door to her so that he could stop paying child support to her. She also told them all that I was trying to take SSs away from her. Um, no BM, no on both counts. 

There was various other histrionic responses from her directed at me, including the latest when she had a lawyer send a letter to DH claiming that "the stepmother's jealousy is keeping the mother from contacting her children" when they were with DH EOWE. She also told DH's entire family that SSs didn't want to come to our house because when they are there, *I* say all kinds of terrible things about her to them. This is on top of the constant harassment she directs at dh. 

So, now, I avoid her like the plague. I avoid going anywhere where she will be. I don't even look in her direction. She cries and screams about this, but when it's just DH, she avoids him and doesn't even look in his direction, so really, pot meet kettle. I do not care what anyone thinks of me, because I do not have time for that kind of crazy in my life. I keep her children safe, we do fun things, and I provide for them when they are in my home. That has to be enough for her. I will not subject myself to harm (emotional or physical) just to make a grown woman feel better about herself. I don't think she's extra special because she had children. If this was truly about the children, conversations about be about them and their needs, not her and her perceived needs, and how she thinks we should all defer to her because she is "a mother." No one has time for that, least of all me.