In this particular person's situation it sounds like there's some backstory that could change things, so I won't comment on that. In general I think it depends on the "conditions" of the original bequest, which may be really informal and open to the interpretation of the person actually holding the items, so I think I would start with a conversation about the items, rather than a request for the items.
Like if it was good china, for example, one could say, "You know, I really miss seeing Grandma's china set out at family gatherings. Remember how Uncle Joe always got the plate with the chip in it, because he dropped it as a kid? Ha ha. Do you think we could maybe get it out and use it at the next gathering you host? I'd really like to see it and I could come over and help you wash it if that's the problem."
And if their reaction is to be horrified and scandalized that you want the china to see the light of day before you walk down the aisle and receive it as a wedding gift, well, you can still make your request, but you know it will be an uphill battle. If they seem amenable to using the china "early," you can segue into, "You know, I've really been looking forward to using the china myself, and I entertain a lot and have a good place to store it. Would it be okay if I took it and just started using it now?"
Kind of asking yourself, and the guardian, if Grandma would rather see the stuff be loved and used now, or she would really have preferred it wait until a wedding--your wedding, another descendant's wedding, etc.. Or if you could use it until the next wedding, at which point you would hand it over to the newly-married descendant if they want it. I could see it depending a lot on the item, too. Rings, for example--if they are nice, "wedding/engagement"-type rings, maybe some family members wouldn't feel comfortable seeing them used as "fashion" rings, even with great sentiment attached; and they probably don't take much room to store, so maybe they'd rather just wait for a wedding. But if it's a set of furniture, that's taking up space/collecting dust in someone else's house, and "you" are buying a new, empty house that the furniture would look great in, maybe they would be happy to give it to you now.