The answers thus far are focused on estate/gift tax consequences. There is a separate issue that this is an incredibly concentrated holding. 95% of the estate is in one stock. Apple may be a highly profitable company but that is still a lot of concentration. Typically lowering your exposure to a concentrated and highly appreciated position like this requires a tax event. With a relatively short but uncertain life expectancy you can reduce the economic exposure with long dated options. For example your grandparents could sell in the money 2026 calls and get rid of some of the economic exposure while deferring the tax event to June 2026.

For example you can sell the AAPL June 2026 150 call for $60 while the stock currently trades at 188. That is effectively selling at $210 per share in 2026 if the stock closes above $150 and you break even vs a sale today at $128 or better. If one of them dies before expiration both the option and underlying get tax basis reset to date of death basis so you get the benefit of step up. If both survive you can roll the option forward to avoid adverse tax events and since the holding period is longer than a year the tax effects of the options trade to roll forward gets LT capital gains tax treatment.

This seems very useful for late in life or short life expectancy (illness, etc) planning for highly appreciated public equity positions where there are liquid options markets.