Am I the asshole for not wanting to use my college fund to save my brother?

Am I the asshole for not wanting to use my college fund to save my brother? My parents divorced when I was about seven because my dad had admitted to having an affair. My mom and I moved out of our house and into an apartment. While my dad immediately moved his mistress into our house because she was pregnant with my twin brothers. My mom was devastated, but she quickly picked herself up for us. She went back to school and as a result she continued climbing up in her career. My mom adjusted better to going from a double income household to a single income and was able to contribute a substantial amount of money to my college fund. My dad on the other hand had a harder time as the single breadwinner supporting a family of four, sometimes five. He manages a second job on the weekend while his wife is a stay-at-home mom. She homeschools the boys and is a full-time caregiver for my brother. My dad couldn't contribute much to the fund as he didn't have much disposable income. I'm going to finish school soon, and I have a good shot at getting into my first-choice colleges. Unfortunately, my brother, 10, male, was diagnosed with a kidney disease which had progressed to where he needs to be on dialysis. My dad is drowning in medical debt and my mom agreed he no longer needed to pay child support. As most treatments either didn't help or stopped working, they are running out of options other than a transplant which is currently not feasible. The specialist suggested a costly treatment for my brother, one not covered by insurance. My dad can't take out a loan due to debt. My dad and his wife sat me down when I was over and asked if I would be willing to let them borrow my college fund to pay off my brother's treatment. They offered to pay the money back, but I knew they wouldn't be able to. My dad's family already disowned him when the affair came to light, and while his brother and sister provided support, it was only to a limited extent. His wife did not have a lot of family, so I do think I was his last option. I really don't want to give them the money because if scholarships don't come through, I don't want to risk my future with debt, and I know my dad wouldn't help if I needed it. In addition, the money technically belongs to my mom. I can't just give it away, and I know she would refuse to give them the money. My dad and his wife are furious at both my mom and I, and they're trying to guilt me into it. I do feel terrible for my little brother. He's just a little kid, and I would feel bad if something happened to him. But I would want to help him with my own money, and I can only do so if I start my career debt-free. I also don't want to risk my own future. Am I the asshole?