if the starter box works ok for your MBX, then it is imposible for it to start your brother's MBX the wrong way round.
I think the gear box in that car is assambled the wrong way round...
... your discription..." start the right way for few sec... then spin the wrong way again..." to me is a very clear indicator that the engine is ok, but the front/rear diff is the wrong way round...
I had my Inferno run in reverse also the first time I venture into ic car.... lol.

just check that the car is assamble the right way... if not... then something weird is going on....