3337th Quartermaster Truck Company was the unit he was assigned to at the time of his release from active duty and may or may not have been the unit he served with in Europe. His overseas unit was probably an element of VI Army Corps, attached to 34th Division. It would have been transferred from Fifth Army to Seventh Army and may have been attached to, or operating with, one of the three divisions that landed and later operated in Southern France.
Re your possible half-brother. If your Dad married while in UA (unauthorized absence) Status, he probably did so without official permission, in which case, there would be no record. In any case, the cost of tracing him would probably be more than you would be willing - or able - to pay. Probably best to leave things as they are.