Lend her money or sell necklace
On the one hand, (comma here) your sister will sell your
's grandmother's necklace. (full stop) On the other hand, you will lend her money but she may not paid it back. If I were you, I'd lend her money because it is better to lose your money rather than sell s the necklace. The Necklace has sentimental value for you and your family because your grandmother wore it. To sum up I think the best thing to do is to lend money to your sister to travel, because it is a pity to sell s the necklace.