1. Do you need to leave school to work with his company? If so, given your doubts, I'd finish your degree. If he's still going strong in a year, you can still come on board while the company is small, reap the benefits of being an early staff member (i.e., stock if it goes public) without worrying about finishing the degree. The problem is that if you jump now before finishing the degree, if things work out, you will just keep on putting off the degree, and eventually credits expire, curriculums are changed, and it can be harder to fulfill the requirements. You only have a year left which is great.
Another option is to see if you can work through the summer and then fall for his company and get some sort of internships credits, and then finish in spring with remaining classwork--this would depend on what you have left to do. Or do thesis work at his company in Spring.
2. Only you know your relationship with your BIL. If you think it'll work, great. If it'll mean you never showing your face at a family reunion if things go sour, think hard and long about it.
3. I hate the NE as well, but if an opportunity were good enough (like being a co-founder at a major company), I'd go ahead and stay. I know it feels like shooting yourself in the foot, but ten years from now, you could move and retire or move and get any job you want, and you won't have regrets. If you move somewhere now and get a mediocre job, you will have regrets. The thing that ties you down is marriage and kids, not a job.
For 15 years I thought I'd be leaving Boston. By the time I left, I'd pretty much gotten settled. Now I'm glad I left, and I have a lot of regrets from being there, but if I'd had a super opportunity to be a founder at a great company, I wouldn't have regrets, I just wouldn't miss the weather.