I read the full version and I would consider walking away from the business. This would:
1) Save your relationship with your father and brother
2) Hopefully help your mental wellbeing
3) Create new opportunities for you (i.e. start your own business or get a job with a competitive salary)
Whether you like it or not, your father has 100% of the equity and is in control. After multiple conversations, you have no equity and are under-paid. You have three choices:
1) Lawyer up - from your comments, it appears you are unwilling to do this. This would probably destroy your relationship with your family and may not even result in you receiving partial ownership
2)Stay and hope that you receive equity. From what I've read, it'll probably happen when your father retires or passes away. The frustration you have, will probably eat at you and damage your mental health. I would advise against this as you may become bitter and resentful. This would damage your relationship with your father.
3) Leave. It sounds like you are young enough and have a wealth of fantastic experience. Go get a job with a competitive salary or start your own business (maybe re-use your code with permission from your dad). You'll probably still get equity when your father hands over the reigns. Say that you want to stand on your own two feet and have something of your own. Try to maintain a positive relationship. If they need you to fix the software, charge close to market rate and point to the fact they are making millions in sales.
Whatever you choose to do, good luck :)