Could I play this with my brother who owns it on Steam?

I recently tried it and the harsh truth is: No, you can not cross play this game with anything.

We tried using the connect feature where the GoG account gets connected with Steam but it did not work out either. I am sure you can only play with other GoG users at this point.

Maybe there is an elaborate trick to make it possible, but I don't know any and the trick has to be really elaborate to the point normal people would be unable to do it.

So to answer your question: It is not possible to crossplay, you either need the Steam version or your brother needs the GoG version. I would have answered earlier but I had no chance to test it out myself with a steam user until now. Sadly no one plays this game it seems.

If by any chance you want to play with a GoG user, I would be avaliable. Just send me a message and we can figure out a schedule.