I really get it, OP, that’s so hard. My FIL died in a car crash 8 months after DH and I got married and MIL depended heavily on him for several years, still does in some ways. She called him every evening and spent an hour on the phone with him. My BIL didn’t get the same number of calls, though he and his DW had a new baby and maintained their boundaries.
I tried to be supportive of the situation, and obviously DH was grieving as well. But my DSis was going through a distressing marriage break-up and was revealing that her husband had been violent towards her, so I was also receiving calls regularly from her, and she stayed with us sometimes.
We were also discovering that I was infertile, a process that eventually led to us adopting our 2 DDs.
I found it so stressful after a while, we weren’t getting any couples time at all and we were newly married still. I had helped my MIL by arranging for the legal firm I worked for to handle the legal stuff, I worked for the solicitor who handled the case, a mistake as it turned out. Because I was finding out that FIL had been to blame for the car accident and MIL didn’t want to believe that her DH could possibly have made a mistake, and she went on and on about how the police had let her down.
It was too much. In the end I couldn’t keep my mouth shut, as I had typed the letters that had spelt out what had actually happened. So we ended up arguing. I think it’s why my relationship with her isn’t great. I knew I was being unfair to resent my MIL, and unfair to my DH, but I needed him to back off a bit and establish some boundaries with his DM, whilst still supporting her.
So you need to be firm with your DM and maintain boundaries with her. Otherwise, you will end up resenting her and that’s hard to overcome. Yes supporting your DM is good, but you need to establish when you’re going to talk to her, for example. And see if your DB could get involved as well?
HTH. 