Should I tell my sister that my mom tricked her?

The night before my sister's wedding, her future mother-in-law gave her a diamond ring and earring set as a gift. Before the wedding, she put it on but my mom told her it doesn't look nice. My mom lent my sister some fake designer jewelry to wear on her wedding and kept the diamond set. When my sister returned the fake jewelry and asked for hers, my mom told her that she would like to borrow it for a while. A few months after, my mom told me in confidence that she pawned the diamond set and because she wasn't able to redeem it, it got sold in an auction sale and cannot be recovered as far as we are concerned. Should I tell my sister the truth and betray my mom's confidence? or should I wait for my sister to remember it and probably distrust me for not telling her the truth earlier?