Money and family just don't go together. Family is family and money is money, mixing them typically ends in a family dispute, drama, or even estrangement. It's important to respect each other's boundaries and not have the expectation that someone is going to just give you handouts only because you're related. It's easy to feel obligated to give money to people you love, but it so often ends up with it biting you in the behind.
The thing is that if someone is expecting you to give them free money, there's a reason for that. They may not be totally responsible themselves, and ultimately they could drag you down into the chaos of their lives, somewhere you don't want to be. If your sister is demanding you give her money, maybe question why she doesn't have any. There's a reason, and as a grown adult she should be fully capable of handling her finances in a responsible way. She could take out loans, or even ask your parents for the money. It's hilarious and dramatic to see the scenes unfold over a piece of paper.