Unfortunately it would have caused considerable damage when your brother has connected the polarity incorrectly!

Components like the ECU/other onboard components and very sensitive, to this end this is why even in normal circumstances you need to be very careful when jump starting to prevent power spikes damaging them!

The first mistake you have made in this scenario though, is to try jump starting, when a battery is flat you need to charge it for many hours, I personally hate the idea of jump starting as it's only good on a partially discharged battery, the first thing to do is disconnect the battery and charge it and then try again.

You may have got very lucky and caused no damage.

It's worth checking fuses as well.

Martin