Hi Klara, feel for you being the grandmother. It’s so hard on the child and those around him.

Screaming is his learnt behaviour reinforced. Because he is learning that it works.
But when he goes to preschool, he will suffer because he hasn’t been modelled nor understands how to diffuse his anger/express himself without yelling.
God, I empathize with his 9 year old sister being put in this position. You’re absolutely spot on ~ she doesn’t deserve it!