First thing is that the insurance co will try anything on, particularly with a younger driver. Call their bluff. A short polite phone call to say their claim is ridiculous and that you will not budge may do the job.
Next help I think will be getting the other driver charged. Call the cop and tell him that you wouldn't have cared either way if they got charged except that now they are trying to avoid paying so would he please have them charged as appropriate. They attempted a uturn over double lines without indicating, stopped in a no stopping zone and drove negligently by not keeping a proper look out for your bro.
If yes, take this back to insurance co for another go. Keep fighting their insurance co for a bit before you go for lawyers or your own insurance.