As others have said - rant away! Your sister is taking advantage of your kind nature. Is it possible your father says he needs to go shopping twice a week because he sees it as an excuse to get out of the house.
It is difficult to say no to a parent but perhaps a quiet chat over a cup of tea explaining your other commitments may make him more accepting of only one shopping trip a week (or suggest he ask your sister to take him)
Water Pollution -“ A National Disgrace”? A case for renationalisation?
BBC Two- Surgeons: At the edge of Life
sticky labels on apples - remove before washing!