Hey Bill,
I'm going through almost the same thing with my parents. They're 81, and my Dad's started falling. I'm the only sibling in the area, so I've put my house on the market and am in the process of moving in with them..luckily, they're both really sharp, but my Dad can't hear, so everybody is talking at the top of their lungs, causing confusion...My Mom on the other hand is sharp as a tack, but is starting to do odd things...Sunday night, I just about got killed when I slipped on the stairs...if one of them had slipped, they would have been really hurt...it seems she had polished the stairs with furniture wax! As I said, odd...
The upshot is that they need help, even just having somebody around to make sure they don't do anything stupid, and somebody's got to do it...if your brother isn't helping your Dad, you are, and that's a great thing...your projects will wait for you...
My poor bike hasn't been ridden much this summer, and I even cancelled a ride a few weeks ago, and I may have to do the same with one next month that I've been planning for a year, but I'm also getting time with my parents that none of my other siblings will ever have...you won't regret it.