Have to be brief as DD has a sky-high temp, so this is just what i'd do in your specific example.

DS1 screams. DS2 begins to cry in fear. I would comfort the baby. I would not speak to or look at DS1. I would talk loudly about "oh the poor poor baby is scared, it's so sad!" and completely ignore DS1 to the point of leaving the room and taking the baby with me. I would make it so that the only reaction to be gained from screaming is that mama focusses COMPLETELY on the baby. I might even go lie diwn for a feed and if DS1 comes to complain explain that you had to take time out from being with them both to comfort the baby because he frightened his brother. The message i would want to send is that if he's going to deliberately hurt or scare another family member then he doesn't get to be around them.

This will work best if you remember to praise good behaviour - it's SO easy to forget to do it enough when you have a high-needs toddler or baby, so with both you will be on your uppers i bet! But if you remember to tell him out of the blue when he's being just GREAT he will really feel the difference when you ignore him totally, and the balance whereby he is told as often (if not more often) when he's doing great as when he's not will help him to learn very quickly what behaviour mummy will react positively to and what will only earn him a trip to coventry.

Bx