Is anyone else ready to throw in the towel?

I, for the most part, can handle Mom's issues. Sometimes, I get frustrated, but I am getting better at being patient. My problem is my brother, who lives with her. He is 71 years old -- 10 years my senior. He has always been known to have a temper (like my father had). He has a terminal condition -- idiopathic pulmonary fibrosis, along with CML, and heart disease. I do a lot for him, as I do Mom. (One of the things I do for him is take care of Mom because I know what he can do is limited.) I also do his laundry, run errands for him, and fix dinner when he is not able to. He got in my face (literally) on Easter Sunday, after telling me to shut up. He doesn't like anyone to question something he says (even if you makes sense). I haven't questioned anything he has done since that day. Then, last Wednesday, he was outside; I was inside with Mom. I heard a gunshot and then heard my brother yell at the top of his lungs (not a good thing in his condition). I went outside to see why he was so upset. Someone in a house behind Mom's had shot off a pellet gun. I won't repeat what my brother said, but suffice it to say, he was more than just a little angry. He came inside out of breath, with his oxygen tank running. He started yelling again at the top of his lungs again (not at anyone, just yelling because of the gunshot). I don't think that is good for Mom to hear, but she knows how he is. In her condition she may not really understand, though. All I said to him was his name in a calm voice, wanting him to calm down (for his and Mom's sake). He turned to me, pointed and shouted to me (at the top of his lungs) "Shut up!" I just looked at him and said, "No." (calmly). He said it again and that he was "tired of my ****." I said (calmly again), "Oh, really?" He said, "Yes" and told me to "get the h out." So, I packed up my stuff and left. I heard him say "stay for her" [Mom], but I left anyway. I came back the next day (and every day after that) to take care of Mom, saying very little to my brother. For days, he had been in a somber mood, with his oxygen on most of the time. He never apologized (which upsets my husband). He really has not been doing well since Wednesday. I am surprised he didn't have a heart attack. That is how bad it was. Then, yesterday, I took Mom to my sister's. She has MS and lives next door to Mom. I have done a lot for her over the years. Unfortunately, what I can do for her now is limited because Mom can't be left alone. I knocked on my sister's door, she opened it and turned her wheelchair around and headed down her hallway, without even saying Hi. I asked her if she wanted us to leave. She didn't say anything. I asked her again, and said something to her about not even saying Hi. She yelled, "I cannot talk!" I guess she was upset about something (she has been having some issues lately). So, we turned around and walked out. Her son said it sounded like it was her normal self, but said she didn't have to be rude after all I do. After this week, I have not been doing well. I feel like I have stopped caring. I want things to be over. And, I feel like I am ready to throw in the towel...