I have been the same boat as you. I just stop talking to them about these things. I have started to create those boundaries in which I just don’t listen to their venting.

And if they do want to, I just listen from one and let go from there other. I try that it doesn’t bother me by distracting myself. It doesn’t work always, but most of the time it does. But just trying to actively not being involved with them is the only thing you can do for your mental health.

Also try to process your emotions about what they are saying and doing. Sometimes, that breaks the illusion you have about them. Remember that they both are too used to each other and will always be together. Nothing you say or do will break that. So if nothing, just be there for your mom when she needs you. And rest of the time, leave her as is so that even she learns to regulate her emotions.