I moved back in with my mother many years back - I didn't have a job, but sensibly one of the first things she did was make it very clear how we'd split the bills, and so on. I had things going on, just as your sister did, but Mum still had to pay her bills. Your dad need to start making things clear - he needs not to go shopping, let her buy the next week's shopping and pay for the fuel herself - why the heck should he fund that? Life isn't easy - she needs to wake up and behave like an adult.