Hello Sarah and welcome to TP, I am glad you have joined even though I am sad you had the need to. I am sure you will find this forum to be very supportive and informative, I find it absolutely invaluable myself.
My late mother used to cry and scream all the time, she could not explain what was wrong except that she was frightened, which I understand is quite normal. My mother only did this in the very late stages, but she was very restless and anxious long before that. She was in pain, particularly in her legs, but could never explain about her pain. She would scream even more when we touched her. It is absolutely awful to experience as you feel so helpless, but I found that massaging her hands and arms and just holding her tight would help, just found it so difficult to do that all day as I also cared for my husband who is totally dependent. It is lovely that you and your brother support each other in the caring for mum. Please remember to look after yourselves too. Have you got any outside support? Take care