Entitled older sister asks younger sister for $2K of her $5K work bonus after having borrowed $3.5K without paying her back: 'I've "loaned" her money probably 6 times in the past 2 years.'

She should never give her money ever again. 

It's never a good idea to loan money to your family. If you really want to give money to your parents, grandparents, siblings, or kids, it should be a gift, not a loan. Giving a generous cash gift does not irrevocably change your relationship forever, but giving a loan certainly can. It transforms the relationship from parent and child to lender and borrower. If you borrow money from the bank, you don't have to go to Thanksgiving dinner with the bank. The bank isn't calling you and judging the things you're buying with money you could've used to pay off debt. The bank is making money off the interest that's accumulating on your debt. The bank will not personally resent you if you fail to pay them back. It will do a whole bunch of other things, but there's nothing personal about a bank loan. 

A loan from a family member is always personal, even when people say that it's not. It totally changes the relationship until the loan is paid, and even then, the relationship might change forever. The resentment a lender feels toward a borrower who hasn't repaid the loan often gets baked into other parts of the relationship. A borrower might spend the rest of their life feeling indebted to the lender, even after repaying the loan with interest. Worst of all, a borrower doesn't have the threat of the bank coming for them hanging over their heads when they borrow from family members. They might feel like they never have to pay the money back.