Before the drip feed, I would say your options realistically are to keep it entirely private or to offer to split it equally. This is because as soon as you get into compromises like sell for £10k, give two siblings £1k each, then you open a way for them to feel entitled to a portion. And logically there is no objective reason for why they have £1k and not £2k or £500 or whatever. (I’m also assuming there aren’t a huge number of siblings). But once you offer any, the only way to guarantee no resentments is to offer a full and equal share. It’s horrible but it’s human nature, particularly if slightly more distant folk are involved like the spouses of the siblings. So either offer exact portion each or say nothing.
Having said that, if it’s value comes from being connected with a famous ancestor, could you actually keep it a secret? Or will they see a newspaper headline about how Famous great-uncle’s War Medal was sold at auction for £££? In which case you’re really in trouble!
But the lovely gift of the engagement ring from the sister suggests to me that whatever about the general and theoretical rights and wrongs, in this instance, it would be best to be upfront. Tell everyone you’ve realized it is worth a lot,you propose to sell it and share it out, or one of them could offer to buy out others if they’d like to keep it in family.
Given what you’ve said, it’s possible the siblings would decline or reduce the money they could take.
And I’m definitely not one of those people who think families should be financially in each other’s pockets with automatic entitlement to loans etc. But with the history of generosity here, it would be good to reciprocate and be open.