Mother is sick.

So she helped Mother with the dishes.

(What does that mean? Did she helped her mother wash the dishes? Or did she wash the dishes alone because her mother was sick?)

It means that she only helped her mother wash the dishes.

If she did the dishes by herself, we'd say,

So she washed the dishes for Mother.

Based on the given information, it means that she helped her mother with the dishes while her mother was sick. In other words, she washed the dishes alone because her mother was unwell and unable to do the task.