When I think of a beginner in a 1500cc motorcycle, at first it sounds kind of crazy. But your brother's C50 is a cruiser, so i'm guessing the 1500 you have in mind is also a cruiser. I can't think of too many bikes with engines that big that are -not- cruisers. Cruisers have a very low center of gravity, and a low seat, so they are easier to handle, easier to manage for their size. Plus the engines are big like that not for power but for stability and stamina.
I read somewhere that half the buyers of new Harley Davidsons were either total newbies or else coming back to motorcycles after 20 or 30 years (raising a family, no doubt). And I wondered how total noobs would want such a dauntingly big and heavy bike. But last year I rented a Harley and I was amazed at how easy it was to manage. It's not a good-handling bike, as bikes go, but it handles easily. I think it was 1400cc but it wasn't really that hot, I mean when I yanked on the throttle it didn't -jump- like my bike does. But it would cruise effortlessly at 90 mph, which is what you want on a bike like that.
Ten minutes on the Harley taught me what the 'cruiser' style is all about. You can't go from 0 to 60 in four seconds, you can't throw it into corners, lean it over to scrape the footpegs on the ground. It's made for a relaxed, laid-back style of riding. It's nimble and manageable around town but stable and comfortable on the freeway. I am not a cruiser person, I don't really like the seating position with the high handlebars and your feet out in front of you. But I could have easily crossed the country on that bike, it was like sitting in an easy chair and watching the scenery go by.
Plus cruisers are probably safer because they're not designed to be ridden on the ragged edge of the 'envelope', at the very limit of tire adhesion, etc. They're just a bike to tool around on and have a good time.
You know what an 800 is like. That's already much bigger than what's usually recommended to start out on. Sit on a 1500 and see how much bigger and heavier it is. Probably not all that much. Could you comfortably hold it up at a corner with your legs? Probably wouldn't be a problem. If the 800 is very easy for you, you could probably 'grow into' the 1500 pretty easily. You sound very confident, which is good. 8^)