DazzlingMilton, OP is* an executor. She's said that twice already, yet you seem to have either missed it in your forensic dissection of her posts, or you're deliberately ignoring it, as it doesn't fit with your narrative of OP being a grabby person who needs to keep her nose out.
OP Are the items that your PIL want to bequeath being used still, or could they give those particular items to the beneficiaries now? As you seem to feel that the two estranged siblings will go and clear out the valuables before the items are able to be distributed, it would be the most sensible option. Are the PIL aware of this likelihood, and if they are, what do they think is the best course of action? If they don't want to give them away yet, perhaps small items like jewellery, could be put in a safe deposit box at the bank. Keeping larger items safe, could mean that only you and DH as executors, have the only keys to the house, until you've been able to distribute the items.
Whatever happens though, it sounds like the two estranged siblings will kick up a stink about something, and they'll start hovering like vultures when the last parent is on their way out. I've heard some horrific stories of children not wanting anything to do with their parents for decades, yet feel entitled to benefit from their estates when they die.
However, I do think you should stop trying to persuade them to have a clear out. I know it will mean more for you to deal with when the time comes, but it's not really your (or your husband's) place to try to get them to get rid of stuff that for some reason they want to keep.