The allocation that you have proposed is rather straightforward and will will self adjust in the event of demise of one or more family members. For a situation with far more beneficiaries, you can create a self adjusting formula. Here's a rough idea of what I did.
I allocated units to family, friends and charities. For example (made up numbers), an immediate family member might get up to 15 units. A cousin, friend or charity might get 1/2 to one unit. The maximum value that a unit can be worth is $20k.
Upon my demise, they'll tally what my estate is worth as well how many beneficiaries are still alive (which determines the total number of units). Divide the latter into the former. If the unit is worth less than $20k, disburse everything. If more than $20k, spread the excess among some secondary beneficiaries (charities) as per another allocation of units.
For some of the recipients, their total allocation is spread over 5-10 years because I don't want them to receive a large lump sum (unless a unit is worth less than $5k and then, they get the lump sum immediately). Because of the formula, everything is self adjusting, keeping the ratio of bequathals in proportion for the many recipients, regardless of who's alive when I kick the bucket.
The only time I have had to update this is if someone really pisses me off and needs to be forgotten (g) or if my net worth significantly improves, making the $20k unit limit invalid. I've only done a couple of amendments in 20+ years.