Mom takes stand with daughters

I have two adult daughters 37&40. For 25 years they have been fighting with each other and venting to me. I always felt as a mother I had to listen. They always seemed to make up after the argument. One tends to sweep it under the rug and the other lets it go for my benefit(so she says). For years this has tortured me. If I try to give my opinion it just makes it much worse. Sometimes I can see both sides of the argument. Sometimes I agree with one or the other, but I never share that because I don't to take sides. This situation has made me very unhappy to say the least. I think about it constanly and worry everytime I'm with them that it is going to happen. One daughter can be vicious and cruel with her words when she is angry and the other being very assertive and confrontive comes back defending her argument in a very authoritative way. Everyone comes away feeling whipped. I as the mother can't stand to wittness this and then listen to the on going venting. I decided to take a stand and tell the latter daughter when she vented to me tonight that I didn't want to listen to this any more because it was too painful since there was nothing I could do to make the situation better. I told her I was going to tell her sister the same thing. She started crying and told me she would never do that to her daughters. She would always have an ear for them and that thay could always come to her. They are only 3&8 now. Up till that moment I felt I was doing the right thing by saying what I did because I wanted some peace and I thought if I didn't hear about the details it would make me feel better. Now I am not so sure because I don't feel better and for the past 25 years I thought "like her" and was miserable. Now I have to say the same thing to my other daughter and I am not sure how it will be received. Would a good Mom have stayed miserable? A tortured Mom