回复：我是个极糟糕的妈妈！诚心向有两个孩子的妈妈学习！

I think the problem is what you said "每次她俩一吵，小的一哭，我一定会找大的麻烦". 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 大女儿 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.