Soooo many questions :-)
By getting into massive amounts of debt
You are choosing to do it despite them stealing from you.
At some point you bear responsibility for how you are treated.
Cut him off. Let him die alone. Not your problem.
I had to do the same with my alcoholic mother.
It was very hard it the moment her antics started affecting the quality of life of my family she had to go.
The money from the house? Since your dad didn't save for retirement, there was never any money to share. If you meant that you're forgoing some of your current income so he can survive, that's a different issue.
I don't even think this is due to a lack of financial education on their part. Feels more like compulsive spending, something a therapist might have been able to help them through had they been willing to talk to one.
Either way, it really sucks that you and your brother had to suffer so much for their failures.
I'll give you a personal example: both my parents _completely_ failed to manage their finances, they kept on borrowing money, refinancing the house 3-4 times.
Since the age of 17, as a young software engineer, I was constantly asked to help with the payments. Now that my mom has passed away, my dad has 0 income, he didn't bother to plan anything for his retirement and thus, we are forced to sell the house and both my brother and I have to forgo our share of the money so that he can survive.
In addition, I _still_ have to give help him financially and he refused to put his money in an investment account so that he can at least profit from the returns.. now he will basically eat every single dollar and then my brother and I have to yet again provide for him few years down the line.
Not only I got no support from them, I had to _on top of it_ fight my way through an uphill battle (they didn't even want me to study CS) and provide for them. They took all the child tax benefit money and ate it.. when I wanted to move at 20, I left with nothing, I just took my clothes.
Sure, parents are wise when it's about "not eating ice cream before dinner", but don't tell me they actually make good decisions.