Yeah you're the asshole. 1/5 (20%) vs 1/4 (25%) is only a difference of 5% that you'd notice.
The fact that she's better off means nothing here, it should not be taken into account. You don't punish people for doing well or reward them for not.
It's your parents estate, nobody gets any say but them.
It sounds like if your sister didn't raise you kids, there wouldn't even be having this conversation because there wouldn't even be the inheritance.
You are 100% an asshole. You need to apologize to your parents and your sister, and encourage your brothers to do the same
Cursed.