How can I handle my sister's harsh criticism of me for the way I care for our parents?

My three children and I moved into my parents home in late 2007. I have been widowed since 2005 and had been finding it increasingly difficult to pay a mortgate and raise three children on my own. My late husband did not have life insurance and there were many bills to take care of.

When we first moved in, my sister who is 11 years younger than I and who is married with no children, sent me a very hurtful email asking if I was going to influence my parents to leave me their home in their will. She said that because I work at a law firm I know how to "fix these things". Our relationship has steadily declined since I moved in with my parents and I find it hard to be around her sometimes.

Fast forward to now, my father (age 80) had a massive stroke in April. He was not expected to survive, but not only did he survive but is making amazing progress in a skilled nursing facility. My mother (also age 80) has diabetic neuropathy and walks with the aid of a walker. I have been the one who communicates with the doctors, nursing home, lawyers, etc. because my sister literally fell apart when my father became ill. My father did a lot around the house, drove my mother to appointments, etc., he and I split the cooking and some chores around the house. Now that he is ill, the day to day running of the house has fallen to me. I realize that since I am living there it is my obligation and I certainly do not mind that, but what I do mind is that my sister likes to make comments such as "you don't take very good care of mom", "are you going to make sure mom eats", "you're not a very warm person", etc. The thing is my mother is perfectly capable of doing small tasks such as making herself lunch, taking her medication, starting dinner preparation, etc. My sister tends to "baby" her and my mother seems to be eating this up! Prior to my dad's illness, my parents were lucky if my sister even called them once in awhile.

I know some others have situations far worse than mine, but I feel very hurt and don't quite know how to react when my sister makes these digs at me and sometimes my children (who are pretty good kids by the way). I have my own issues to deal with -- one daughter in college, a son starting college in September, and a daughter with learning disabilities going into the 10th grade. I help my children out with tuition, books, travel expenses, etc. so moving out is not an option right now.

Any advice would be greatly appreciated as I am on the verge of tears right now.