I guess the best you can do is try to project cash flows and returns. One positive, your parents aren't emotionally attached to the place and pressing you to keep it going. Other "soft" issues are is your brother going to help? Is it OK with you if you do a heap of work and he also benefits, as heir or part owner of LLC, if that is your parents wishes? Do you work well together as partners?
So for sale, proceeds minus capital gains and depreciation recapture taxes, then project investing the cash in x fund for x years.
For repair, cost of financing (if you can get it), cost of construction plus contingency factor, cost of your time managing, compared to rental cash flow and appreciation. If a silent partner, put in some probability factor that they flake on you or cause you stress, in addition to them getting their share. There's that vision of keeping a "hot" property vs. the potential that the repair effort will reveal more and more hidden costly problems, gradually sucking up all your time, money and sanity . . .
I agree you might want to talk to an attorney about a trust, dependent on your parents financial situation, to protect the asset now while they are healthy. If either or both got seriously ill, do they have resources, long term care insurance? If there is any possibility they could end up financially wiped out and looking at Medicaid for home care or assisted living? (Medicaid has a five year look back period. Medicare doesn't cover that stuff.)
And a final, I guess, Mustachian question, even if the numbers work, do you have "enough" without going through the repair/rehab effort to make the property viable. In other words, what is your reason to want to do this -- Is it, "Damn, I can't stand the thought of somebody else waltzing in and grabbing that upside," or "I'm really excited about and interested in rehabbing and managing a sizeable property, what a great experience and learning opportunity," or "The return outweighs the alternative of investing the sale proceeds so much and it is essential to cover college costs for my and my brother's kids (or provide a living for the entire family, or . . .)" Only you can answer that one.