Bad relation with my Mother. Any advice?

I am a mother of a son and a daughter. I try my best to be a good mother and a good daughter, but very often I don't know if I have done the right thing. My pain come from my bad relation with my mother. I know I should love her but to be honest, I do not like her. I think she is mean, dishonest, partial and stubborn. She has a tongue of stagger, often hurting family members. My father got lung cancer and is in bad condition. He cannot speak, cannot eat with mouth, cannot hear anymore. Instead of understanding his agony and giving intensive care to him, she complained very often about my father' bad temper and doesn't want to do housework. My sister ( who has more time than me since her children are in universities in another city) gives most care to my father, cooking,washing, and feeding. I try my best to offer some help, but my help is limited and mostly is financial because I have two children to care, and my husband often goes out of town for business, and my workplace is very far away from the town center where my mother and I live. Last week my father sent me a text, asking me to ask leave from my work so as to help my sister and my mother with housework. To tell the truth, I was somewhat unhappy about this, because I think my sister and my mother can handle the housework, there is no need for me to ask leave from my work. So, I texted my father and told him about my difficulty. He is understanding. Anyhow, I do not want my parents and my sister think I am avoiding the responsibilities I should take. Before my father's text, I went to see them everyday after I have picked up my daughter and stayed for an hour. While I was there, I would talk to my father by writing, and gave some help. My sister would do the cooking, because most of the time she has come back from work and has begun cooking when I came there. After my father's text, I managed to go to my mother's in the afternoon earlier than before and helped to do cooking and some cleaning work. I went there at about 4:00.As soon I got there, I started to work. I cleaned the floor, and I prepared all the food for my father and cooked the meal for my mother, my sister, and her husband,who sometimes comes for dinner but not everyday. Then, I left at about 5:00 to pick up my daughter from kindergarten. While I was doing all these, my mother was sleeping. I could feel that she was pretending sleeping,because every time I got there and was unlocking the door, I could hear the sound she made when trying to go to bed quickly. She would sleep on bed and didn't get up until I left. However, I didn't say anything about this. Yesterday, I went there at about six after I have picked up my daughter.My mother has got up. My sister has not come back from work, and I started to cook as soon as I arrived. I was in a hurry because I had to go back home to cook for my son, who would be back home from school at 7:30. My mother told me not to cook too much because she was not sure whether my sister' husband would come for dinner. When I finished cooking, she came into the kitchen and said very angrily to me that I have cooked too much because my sister' husband would not come back for dinner ( while I was cooking my sister came back and told her that. I din't heard this. But I know my problem was I should have asked my sister when she came back about this but I didn't. ) I explained to her it was not too much. She wouldn't listen. She went on blaming me that I cooked too much on purpose. She said I cooked too much because I didn't want to cook for them. She said I was showing my unwillingness in this way. She said I cooked too much because I want them to eat leaf over tomorrow. She said I didn't want to cook for them because I seldom talked when I was there. She said she would not eat the meal and asked me to take the meal I have cooked home. I was very angry when I heard this and lost my control and yelled at her. I said she was doing too much to me. I said I was not as bad as she had thought. I said she has wronged me. I said I didin't talk because I didn't have time to talk. I was so angry that I shouted at her. I did all this in front of my four year old daughter, which put me into greater pain, because I have always wanted to set a good model to my daughter of being a good loving daughter. I failed again. Similar things like this happened several times.Whenever I am in such bad situation, I am on two minds, which is an indescribable torture. On one hand I am bothered by the sense of guilt. I think I should try my best to make my parents happy and take good care of them and tolerate everything. On the other hand, I do think my mother is too demanding, and too mean and unreasonable. Moreover, I am afraid my kids might think I am not a good daughter and will not respect me. Whenever I was in conflict with my mother, I need several days to recover, to forget all the painful feeling. I could not concentrate on my work, kids and husband. Please help me. Thanks.