回複：我是個極糟糕的媽媽！誠心向有兩個孩子的媽媽學習！

I think the problem is what you said "每次她倆一吵，小的一哭，我一定會找大的麻煩". Some time just because you see the younger sister is crying, doesn't mean the older sister did it. You only see what happened in the moment, not before then. That's probably why 大女兒 felt you didn't love her anymore, because sometimes it may be not her fault, but she still gets the blame, just because she is older. Sorry can't type in Chinese, hope you can get my point.