You need family counseling. Of course it's hurtful to your son!! I get it...she wishes she was an only child. My daughter says that just joking with her brothers (yes, they know she's joking and fire something funny back at her...they're all adults and can take a joke) and she would NEVER exclude them from something like that. Even at Christmas when she visits, she makes sure to include them and their children with the Santa portraits. She even went so far as to recreate a picture with Santa from when they were little. Are they close? No, not really, but they know they can count on each other during the bad times. This "improvement" is NOT an improvement in any way. Yes, it will be sad not to be able to hang it, but it is sad that she excluded her brother. Please get some family counseling so you know how to deal with this properly and as a family unit.
You COULD help her, but you are not obligated to do so. You could compromise and offer her "X" amount and tell her that's it, no more, and pay it directly to the college so she doesn't change her mind and blow it. But again, you are not obligated to do so. NTJ.