I have a different suggestion, something to try for 2-3 days.
Ignore the scream. As in, do not respond at all, don't even flinch, pretend it didn't happen. She is doing this to get your attention, and it's working every time. I know it's not good for your infant son, but it's happening anyway, and isn't causing long term damage besides getting him upset. If his crying tends to be short-lived, let him soothe himself after his sister upsets him. If he's really distressed, do the minimum you need to to calm him, but try not to make your daughter jealous about it.
Later, when your children are behaving well together, acknowledge how nice they're being to each other to encourage them.
I got this strategy from a book called Positive Parenting. At this age, punishment like time outs has limited effectiveness, but ignoring her bad behavior will work wonders. After all, she wants your attention. Then positively reinforce the behavior you do want from your children. You want to teach your daughter how she SHOULD be behaving, so in my opinion, yelling back, hitting, etc., are not good options.
This method really worked for me and my 14 month old, who used to scream the instant she wanted something, in a matter of days. No more screaming (well, very little).
And I do recommend, if at all possible, spending daily alone time with your daughter. This is such a big change for her. Your infant isn't going to remember this time with you, but your daughter will.
Anyway, I really really think this will work in this case. Two-three days and you should see a difference. If not, then you can always try something else!