Same in the regard of having two pistons oscillating side by side...much like a Sportster vs Rebel both have two wheels, things get very different once you get past that trivial aspect.
Off the top of my head....
> square bore/stroke vs large bore/small stroke
> 2 valves/cylinder vs 4
> 1 camshaft vs 2
> same plane crankpins vs offset crankpins
> one carb vs two
> five speed vs six
> air cooled vs liquid cooled
> 9.2:1 compression ratio vs 10.2:1
> max RPM = peak power RPM at twice the displacement
I'm sure there are others that escape me but you get my gist.
2004 Harley Davidson Sportster 883 Custom Specs:
Bore x stroke: 3.0 x 3.8 inches <-- Not what I would call a stroker, but the Harley does have a "longer" stroke to bore ratio.
2 Valves per Cylinder: (2 = same as Honda 250)
1 Single Cam (Same as Honda Rebel 250)
Fuel system: One Carburetor (Same as the Honda Rebel 250)
Gearbox: 5-speed (Does your Rebel have six gears? Mine has only 5)
Air Cooled (Same as the Honda Rebel 250)
Lower Compression Ratio 8.9:1 vs 9.2:1 for the Honda Rebel 250
Honda Max RPM 8,250 rpm VS Harley 883 Rev Limited to 6,300
Weight: 574 (vs 331 pounds for the Honda Rebel 250)
HP: 41 Vs 17 for the Honda Rebel 250
Top speed: 103 mph <-- 80 and above = Scary
The Honda Rebel has 17 HP (could always use more, but sufficient)
Top Speed 83 <-- Scary
My 2004 Harley is faster and has more torque, but in my humble opinion is quite comparable to my 2004 Honda Rebel 250.
Your Mileage may vary, but in my experience, the Rebel is more than a just younger brother.
I love the Rebel.
I also love the Harley.
Both of mine have a windshield and have the same seat/backrest/rear rack and look like brothers.
But, as you pointed out, they really are different.
Not as different as you might expect but I have to agree, they are different.
The blue line on the chart is the 883 STOCK before mods.
Not my 883, but just to put things into perspective..