Older sibling danger for the younger ones

We've been on a vacation and my oldest son. (Half brother to his younger two siblings)

Me and my wife are at the beach and watch the kids play. My oldest son swims out with my middle child. He's a good swimmer for his age and we are close enough.

My middle child is exhausted because they been swimming further away with an air matress. He yells his brother name "I'm exhausted, I can't anymore"

My oldest one swims away leaves him almost drowning. Me and my wife couldn't beliebe my eyes.

I can barely speak to my family about it, I work with kids and teenagers and still wasn't able to resolve this situation properly. I know he's jealous because his sibling grow up with their dad and he's not. But I was almost close to never let him near his siblings anymore.

Anyone encountered situations like that or experienced things of that nature?