Serving one's mother in-law and speaking kindly to her

I want to ask that I take care of my mother in-law who is unable to walk because of a weak spine. But she understands everything. She is mentally alright. But she shows that she doesn't understand how she urinates or other things. So I warn her for doing all this. Am I doing a gunnaah if I warn her? Please advise me because I find it very difficult to do all that work.