Screaming whinging child - how to handle?

[Letmesleepalready]

DC5 keeps snatching things off DC2, has huge tantrums every day, multiple times, and is never content. I don't know how to deal with the behaviour without becoming a screaming mess myself.
They always want what their sibling has, and have meltdowns every single time things don't go their way. It's exhausting having to deal with that constantly. I don't know how to break the cycle. Any ideas, please?

Is the issue that your 5 yr old keeps taking things off your 2 yr old?
Is the 2 yr old playing with what the 5 yr old thinks is their own toys?

Yes, the 5 year old will always want and/or take what the 2 year old is playing with. It's not always "their" toy, but invariably they will have no problem playing with the 2 year old's toys, but kick up a huge fuss (snatching, screaming and /or tantrum) if the 2 year old tries to play with pretty much anything. And when they do let the youngest one play it's only because they are "bossing" them around telling them what DC2 can and can't do.