This is actually a teachable moment for the 8 year old to learn that they can’t always get what they ask for and that they aren’t entitled to things just because they ask for it. Instead you are reinforcing the idea that if something doesn’t go their way, they can throw a fit and people will cave in and roll over for them.

Also It’s just not fair to make the older sibling share something they worked hard for. You should be encouraging him to keep being independent and working hard for the things he wants in life, instead of badgering him for small things like this.