ETA: wrote this then sat an hour before posting while everybody else said mostly the same thing. Sorry to pile on redundantly.
As a matter of law you must follow the terms of the will.
However once the probate is complete and you and sib1 have deposited the estate’s payouts into your accounts, you’re each free to do whatever you want with what’s now solely your own money. You can burn it, buy booze with it, give it to the local cat orphanage, or give it to sib2. In any combination you prefer. You and sib1 have no need to do the same thing. There are tax consequences both positive and negative to bear in mind for any gift to a charity or to sib2. Burning it and drinking it are tax-free actions.
So far so easy. Now comes the hard part.
Your Mom and Dad will never know nor care what you & sib1 do next. You two are the ones who have to live with each other, with sib2, and with the consequences of your actions. You don’t mention if sib1 or you are married or have kids. Which if so add additional interested parties into the mix.
Note that sib2 also has to live with the consequences of her actions now. Having unilaterally chosen to open this can of worms, she now must be prepared to eat them. IOW, the mere fact she’s asked does not mean the asking was/is reasonable.
Ultimately this is about you and sib1 buying a different future from sib2. Whether that’s knuckling under to blackmail or being a stand-up fair-minded person is for you each to decide. It’s also worth considering if that different future will stay bought or if sib2 will say “Thank you” quickly followed by “What have you done for me lately?”
I will suggest that to the degree sib1 & you do similar things, that future will be better.
Good luck. Seriously, not snarkily.
Story time:
I nearly had a similar problem wherein my wife & her one sister were treated very differently in their mother’s will because reasons that made sense to emotionally blackmailing people from the 1920s. But that made no sense to my wife, her sis/my SIL, or to me.
Anyhow, wife and I had a standing plan for decades that when Mom died and after the estate was settled we’d make sure sis/SIL got 50% of Mom’s payout. Because that was unequivocally the right thing to do. Both for moral justice and to avoid putting an utterly unnecessary and unwanted wedge between wife & her sis. It was helpful that wife & I were much wealthier than sis, so the practical negative impact on us would be much smaller than the practical positive impact on her. So Win-win sorta.
Leaving out a lot of irrelevant detail, my wife died first so the issue became moot. But we had a plan.
As this might apply to the OP he needs to consider how differently $X will affect sib1 and himself as well as whatever families they have. Sharing the pain equally and sharing the dollars equally are not necessarily the same thing. if indeed anything is to be shared w sib2 at all.