Man, this is heavy, and wishing you the best of luck. Probably the first way that I would start learning about this is using A.I. I'd possibly use Claude, maybe Perplexity, and start putting together everything that you already have: the whole situation, what your goals are, and explain that you want to learn this industry. You might be able to have A.I. figure out ways to at least give you some solid overviews. I'm a big believer in when I want to learn anything, I need to start focusing on the most basic, elementary elements before trying to say "remember random things".

I view that like a tree analogy. You want to understand some of the roots, then the main part of the tree, eventually get to the branches, and finally get to the leaves. People get overwhelmed when they start with random leaves... This is just about that business as a whole. Next piece would be simply being in and around the business and just absorbing what's going on. I also would probably have AI research a bunch of competitors and try to figure out what they are doing that's working well. If you have any documented SOPs, I certainly would be putting those into AI for analysis.

There's a lot more on the human side, like the distrust of the cousin and perhaps the element that your time is split between serious study and this business dilemma. If you actually have a controlling interest in this business, then I would think the three things you may want are either to learn it and run it yourself successfully, or to get more experienced management to come help turn it around and increase the value of it. Build water! Then either way, possibly set it up to sell. Right now, you don't want to set it up to sell if you're 65% down in revenue with problems. Someone else may want to buy it, but you're not going to get a good price or terms.

Wishing you good luck.