Sorry for the loss of your Brother-in-Law. My condolences to your Sister.
Post an inventory list and as many pictures as you can manage. I have seen other threads asking about value of used equipment and apparently, we LOVE to price used gear. It's kind of like shopping with other people's money.
Most used equipment follows the "50% of new price rule", but reloading gear is usually 75%. Being able to sell an entire lot is worth discounting down a bit, even as far as down as 50%.
Primers still in original packaging usually is worth 100% of new ($3 to $5 per hundred last time I checked).
Gunpowder is $18-$35 per pound last time I checked, depending on which powder, but if not in original, sealed containers is worth little to nothing, depending on your tolerance for risk.
Bullets are worth what you are willing to pay. They don't age, spoil or lose potency. But if they won't fit your firearm or fill a need you have are no more valuable than the cheapest you can find in a store.
Like I suggested. Post pictures and an inventory list.
If you give your sister the web address of this thread, she can see the valuations and eliminate any intra-family negotiating, which can be emotionally draining in a time of loss. Let us take that burden.