I couldn't live with someone so different from me, not just in political persuasion but in ability to discern facts. That could just be me though. It's a shame.
You plans for the lake house sound great, but I have one caveat - what happens to you if your brother-in-law bails? Would your sister be able to buy out his share of the house? Is his 1/3 or 1/4? Make sure you have EVERYTHING for EVERY contingency spelled out. What if you married a guy who didn't get along with your BIL? How would that be worked out?
(Confession - I've thought idly about the same thing with my sister. If she lived closer - we each own a nice house, but it we combined forces, we could live in a mansion! However, I wouldn't want the maintenance or expense of that.)
