You and your sister have different ideas on how to care for your mother. When you use statements that start with “you should,” you are sharing your opinion of what should be or needs to be done. We usually say “you should” to others when we think we know what is best for them. We may feel frustrated when they disagree with us. By saying, “I should” to ourselves, we may end up feeling depressed and angry because what we “should do” is not what we really want. A caregiver might say, “I should not even consider putting Mom in a nursing home. I must carry on.” This approach does not lead to an open dialogue or exchange of ideas. It is important to try to understand your sister's perspective and to communicate with each other.
Consult the article Caregiving Is Different for Everyone to help you understand better how your sister is viewing her caregiving journey.
You may also find more more information at Managing Anger and Frustration.