Who is in charge here? As long as you give in, he'll keep throwing fits. Here's the rule..you have to drink some water FIRST and then you can have juice. Fill it up half, not all the way and show him it's not even full. THEN, he can have juice. Water should help fill him up and quinch his thirst so that he won't have so much juice either.
If he refuses the water, he CANNOT have juice, NO matter what! If that means he cries himself to sleep, then he cries himself to sleep. This is NOT being mean. This is making telling him that you are the parents and you make the rules. Kids have no respect for parents who always give in. Trust me on this.
If he wants to scream, there needs to be a designated place for this, NOT in the room with the other brother or in your room. It can be in a den, library or some place where he can put a pillow in the corner or on the floor and cry it out. Might want to put a stuffed toy, book or something else there, too.
Ask him which is more important..having water or nothing at all. Try letting your juice supply literally run out. There will be NO OTHER choice. Stop giving in. Need to tell him the sugar from the juice isn't good for his teeth, either. Set boundaries. You can have juice first thing in the morning and ...... Setting boundaries are part of life lessons.
I do understand your situation, but the longer you give in, the harder it will be to change it. Also.....make SURE you're giving him adequate fluids throughout the day. It only takes 3% water loss to be in dehydration. If he is that thirsty, maybe he's not getting enough throughout the day.
There is also a book called RAISING RESPECTFUL CHILDREN you might want to check into.