Funny as I am wearing the one piece of jewelry I received from my husband’s family after my mother-in-law died twenty years ago. The bulk of the jewelry likely went to her 2 daughters and not the sons, even though they have daughters.
My one brother-in-law was famous for saying that “dad promised me” different things within the estate (artwork and some antiques). It was very challenging for the two siblings that were executors to figure out the fairest way to value what he claimed and it was very stressful because he didn’t want it “counted” against his share. I don’t think they “back figured” stuff he had already taken/been given.
My mom had very little jewelry. I believe she offered to both me and my sister, none of which was worth much.
I think it will be very difficult to get your sister to agree that the jewelry she already has is part of the estate. However, if it really was worth a lot of money your brother may push for that and perhaps sister would bring out some of it to be valued. Unless the jewelry is very valuable, not sure that it would be worth fighting over for a few thousand dollars each (value divided by 4), especially if nobody wants the jewelry to be sold.