Only one way to find out - go buy a Pugsley and challenge your brother to a race!
I'm pretty sure I'd crush him, he's not a very good skiier
(although way better than me). He'd crush me if we were both on bikes though.
I think alot would depend on how firm the snow is. If the Pugsley starts to break through at all, it's game over.
I think with skate skis specifically, the track has to be groomed pretty well, and would be fine to ride on with a bike. From what I could subjectively tell when I went skiing, i wouldn't break through on my winter bike which only has 70x35C tires.
I'm pretty sure you get lots of crossover with cyclists and skaters. I remember seeing them show one vancouver resident (woman's long track) showing up to the olympic stadium on her bike.