I would have a conversation with yourself about what you want the end result of this to look like. What's the best outcome here for you? What's the worst? What about for your parents? And I'm not just talking about the cash: is it worth it to press this if it means losing your family?
One can consult a lawyer without bringing any action, which could be invaluable in providing an objective, outside opinion.
Sadly, I know from experience that family ties are no insurance against mistreatment, financial or otherwise.
I've already decided it isn't worth it. I have no plans to lawyer up in this situation.
Even if you don't take equity as the payoff to the time you invested in working on this project, if you were really unpaid during all those years working on it, you likely have a strong case under employment law if nothing else, if you can document time you spent on it. It sounds like your father really took advantage of your naivete here. I can't imagine doing something like this with my son. Even if I didn't think his involvement warranted equity, paying him for working on it would be done without question.
There's a lot to learn here, obviously. One thing I'm picking up on reading your pastebin is that this is a company that grew to at least several employees, and operated without your involvement for at least a few years. There's a lot that goes into building a company, and just being around for the start of it doesn't entitle someone to half the company—this is why vesting schedules are a thing. So its worth looking hard at your expectations and being brutally honest with yourself about the role you played in the formation of the company. At the end of that, though, if you're able to honestly conclude that your role was absolutely integral to the formation of the company, you should stand firm in demanding to be compensated for that.
Absent an employment or contractual agreement that makes your code a work-for-hire, you own the copyright to any code you write. If you don't have any agreements in place with the company your father started with you, then you still own the copyright to the code you presumably wrote during the early days. Even if they had developers who later built upon that, their copyright is derivative. They can't do anything with it without your agreement, including sell the company, the software, or really even deploy it if push comes to shove. I'm assuming you can prove your own copyright in the form of source control.
I've been through exactly this situation with a client, and if I hadn't had that card to play I'd have gotten royally screwed dealing with them. I don't know if I ever really got as much out as I put in, but it made me almost-whole. (and you can reach out to me if you want the full story there, I'm not going to put it on HN.)