One need not share personal funds with one's siblings.
Justine is visited by her sister Cindy, who wants to borrow her credit card to make a small purchase.
Justine wants to respond to her sister.
Justine tells Cindy that she is not comfortable lending out her credit card to anyone, even a sister.
Justine gives Cindy her credit card with the expectation that Cindy will eventually pay her back.
Cindy understands Justine's reason, and she looks elsewhere for the money.
Cindy runs up a huge debt on the credit card and never pays Cindy back.