Unfortunately, I don't think there is really any way for this to be "fixed" other than to fix your own response to it.  

You say that there are some underlying tensions.. but also that you both work around them.  I don't think she is using the wording in any outright attempt to get under your skin.  That's how she talks about her mom and references her in conversation likely.  

As long as you are secure in your relationship with your DH (as in your relationship between you is good).. and as long as she is civil to you when you are around each other.. I would try really hard to do less disecting of the meaning of her phrasing... bringing it up is likely to air a lot of grievances that probably would be better left alone.

It's just a habit, I would not let it stress you.

I know people that refer to their parents that way when talking to anybody, which I think is really strange.

Funny sidenote: My SD's BM lives only a couple doors away from her, BM is getting on her nerves so she now refers to her as, "the neighbor".

The first time she did it, she was telling DH that she was having problems with the neighbor. DH, being sympathetic (and forgetting that BM lived practically next door), said, "Yeah, having issues with a neighbor really sucks. What are they doing?" Then, she told him that she was talking about her nosy mother! DH and I just busted out laughing.

She is now very excited, as the neighbor is moving in October.

We often give the benefit of the doubt when people say something that we find rude or minimizing.  Ya know like we think maybe they didnt mean it, thats just not like them..

BUT what about people who are repeat offenders? They are known to  undermine, make passive aggressive statements, just all around negative vibes towards us?  THEN the benefit of the doubt goes out the door, and we are left with oh great snot nosed is at it again.

Which one is your SD? Someone who is generally empathetic and kind, OR a meddling lil B who has in the past to  try and minimize you and her daddios marriage?

OR maybe she is a bit of both. YOUR gut knows

ETA:  I would let it go. 

My skids do it and my own kids do it. (Keeping it vague for privacy but there's a lot of them between us). 
 

I would find it odd too, SD has always referred to BM as 'my mom' not mom around DH or I.  I think it also depends on how old they are when they divorced, SD was around 7 or 8, my kids were even younger, so the use of 'my dad, my mom' or 'your mom/your dad' from us when referring to them catches on.

If they live the marjority of childhood w/parents together, I can see where using mom/dad only gets more ingrained when speaking to the other parent, does that make sense?

It may be aggravating but it sounds like she just assumes that Mom means her Mom.  I would just ignore it, it doesn't seem worth making a stand over.  And if you did, it would make you seem like the  unreasonable one.

I think she is referring to Mom in a sentence the same way she would say a person's name. "Mom went to the store", "Laura went to the store". 

I wouldn't let this take up head space. I get that it is irritating but making something out of it isn't going to do any good for anyone. 

I would not take offense to this. My own bio kids do this when talking about their dad. They reference me the same in front of him and his wife. There is no insinuation to "a still existant parental togetherness." Regardless of divorce, our titles to our kids will always be MOM and DAD. My daughter often sends snail mail addressed to MOM. LOL

My DHs kids do the same reference and it's never bothered me. Regardless of what I thought of the BM, she is their mom.

From 2021.  From what you mentioned, this one is needy and you were abused by the older one, and forced to watch as your husband posed in wedding pics with crazy ex. so you have a form of ptsd...and are really sensitive.

This one is not a hill to die on.

I think this would bug me in the context of DD. If SS were calling his mom "mom" instead of "my mom" and it started to confuse/influence DD then I'd have an issue. Otherwise, it's her mom and perhaps your DH can respond with "your mom"... I think the bigger question is- why is she talking to your DH about her mom if they haven't been together in years? Has he communicated that he really doesn't care to hear it?

But I don't think your SS is doing anything to intentionally bother you.  It's probably just a matter of that's how she always talks about her mom's house, mom this, mom that, etc.  But OMG, I do understand how it can be grating.  My advice would be try your best to just ignore it and focus on something else when it happens to try and keep your own sanity.  

My skids refer to BM as Mom when talking to DH. They refer to her as "my mom" when talking to DD2. My DH refers to her as "your mom" (when talking to skids) or "their mom" or "skids' mom" (when talking to others).  I'm not bothered by the kids calling her Mom to DH. I don't think they mean anything by it, and it therefore doesn't bother me. I can see how it could bother other stepmoms, though.  This is one I'd let go - I think it could sound petty if you were to bring it up. 

At least she's not referring to her as "Mommy."

At the time and had cut off contact and disappeared for over 5 yrs. in that time hubby met me and we got married and had 2 kids together aged 1 & 2.

what was messed up was bio mum had cheated actively whilst married to my husband and married the affair guy who left his wife to marry my husband's exwife days after divorce had come through whikst bio mum was faking this whole poor me singly mum bs.

sd's kept inserting bio mum and stepdad to every conversation and even inappropriate things like biomums marriage probably won't last long which translates to "current hubby has figured out bio mums shenanigans so she needs to line up my husband since his salary increased 10 fold since he divorced biomum"

everytime bio mum and stepdad were mentioned, it went through one ear and out the other, many times hubby made no comment whatsoever and it fell on deaf ears.

3 visits in with sd's and i refused any more visits. It was absolutely torture and the level of disrespect m. My husband knew what was going on but too afraid to deal with it.

i told him i was done with his kids and there would never be a day where we would be together agaim.

it took my husband almost 1.5 yrs to finally tell off eldest sd who was almost 25 that he'd had enough of these non stop rants. That he couldn't understand why she even thought it was appropriate to insert bio mum and stepdad into every conversation going on with us??

hubby told her that he has no care or concern for bio mum and rightfully so since she is married to current hubby and therefore any issues she has are her and affair hubbys problem to deal with

my husband made it clear that his priority was to me and our 2 kids together and bio mum/stepdad didn't supercede that. He told sd it was entirely disrespectful to me as his wife and mother of his 2 kids to have to put up with this nonsense.

sd knew what she was doing and responds with "sorry of i did anything wrong" then went on a sulk fest. Frankly my husband is glad to be rid of bio mum. He made it clear to sd that i am his alpha female, his current and future and not bio mum and he never wanted to hear anything about her ever again