How should I handle my sibling & partner stealing and selling my valuables?

About six months ago, I found out that my younger sibling & their partner took several valuable items from my room without my permission and sold them. I’d been holding onto these items for about five years and planned to keep them for another ten because their value increases significantly over time. They sold everything for around £250–£300, even though the items were worth about £800 total. They admitted to taking and selling them at the time, saying they needed money for a holiday they went on. I was annoyed, but I let it go because they promised they would pay me back. Fast-forward six months: I recently checked my collection and realized the items were still missing, which reminded me of the situation. My sibling and their partner have since broken up, and neither of them has made any attempt to repay me. When I called them, both denied remembering anything and suggested I must have misplaced the items — even though they were the only people who knew about them, and I’m certain I didn’t lose them. So now I’m stuck. Should I demand they pay me back? Should I let it go? How would you handle this?