What am I doing Wrong?

My husband and I take care of my 88 year old mother, who lives with us. 3 years ago she took sick (osteo-artitis). My husband is retired, I'm still working. My husband is her primary caregiver while I work. When I am not working, I try to take over to give him a break. We had a good weekend until this morning. We got mom a hanging plant for mother's day. We put it in the same spot we put her mother's day hanging plant. Since we put it there she's complained that it doesn't get enough sun (despite it's the same plant, and same spot we always put her mother's day plant for years). Finally today, I lost my temper. I had enough of her nagging about the plant. My husband finally moved it and she's sitting on her bed pouting. We do all we can to make her happy and she acts like a child. My brother came over to visit and she completely ignored him. Now my husband is mad at the way I handled the situation. I feel like nothing I do it right in this scenario. I'm spent.