I'm sorry you have gone through all of this and very proud of your accomplishments! Like you I put myself through school (part way for me) and always worked.
There is good news and bad news.
The bad news is that the solution is not going to be how to make your dad do right and provide money for your brother. Because he won't. The solution will have to be that you tell your mom the truth, and your brother the truth, and work with what you have.
The good news is that if your brother works very hard, he can still go to college and have a good life. And even if he doesn't go to college he can build a good life with a trade etc.
My parents never promised college money or gave any. I managed to get one year of art school on full scholarship, then one more semester with loans, then the debt was building and I was too broke to continue so I quit to work full time. I had some very poor years, but that drove me to work my way up some ladders from entry level positions, and by age 26 I had moved to NYC, quit my job to start my own company, and I had my own clothing line for years before leaving work to have kids.
Whatever opportunities you have you can build on and so can your brother. Your dad should be held accountable for breaking his promises-for whatever that's worth. But don't depend on him and don't let bitterness hinder your future successes.