That's complete nonsense. Obviously you dont handlea yopung baby if your hands and clothes are really clean, but the child must build up natural immunity to infection - providing of course that the child is healthy in the first place. At what age would your brother and his wife allow other people to handle the child ? One, two five ? However, it is the parents decision and if your sister-in-law yells at you, the you need to accept that. The other factor though is that if the baby doesn't get used to being handled at an early age, he could easily develop a complete aversion to people he doesn't know later on, and that's not good for overall development. You need to respect the parents wishes first and foremost. After all, he is their child.