Is it wrong to not want to hear my mom rant/vent to me about her deteriorating marriage?

Let me start off by saying me and my mom were never super close. She has her beliefs and I have mine. She got into a very heated argument with my step dad earlier and I told my sister about it, my sister told me to go talk to her to see how she’s doing and I told her that I wasn’t comfortable doing that. She then told me that I needed to be there for my mom and I said something like that’s between them 2 (my mom and her husband). My sister got super pissed and said that I should be there so that my mom could vent to me and that I’m immature for not talking to her. I love my mom but I don’t think I should insert myself in that situation and my sister doesn’t really seem to understand that. Is that really such a bad thing?
Is it wrong to not want to hear my mom rant/vent to me about her deteriorating marriage?
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