This with bells on.
If it was you getting 33% and your sibling getting 16%, I'd have been minded to consider it, for family peace, but for cousins? Nah... do what your grandmother wanted. (I may be biased though as my cousins, through an inheritance or two, were found to be moneygrabbing bastards!)
They don't think it's fair? Well, clearly your DGM didn't think it was fair that they should financially benefit when you and your sibling were clearly much nicer to her when she was alive!
When my DGM died, her will stated that her estate go to her two children equally. My DM's sister had passed away, so her share went to her 5 children. My DM is still here, so got her share as planned.
My DM was Executor of the will and each of the 5 other grandchildren regularly contacted her asking for an update and for when they were going to get their money, despite not having ever visited my DGM in her care home, or in the years before she went in to the home.
I've never seen anyone move as fast as I did on the day of my DGM's funeral when my DM asked them at the wake to write down their bank details so she had them ready for when probate was granted.
As soon as they all had their money, my DM hasn't heard from them again. It's been 6 years now, so there is definitely no love lost there from my point of view.