Our ❤️❤️❤️❤️ are broken

We have 2 littermate dogs. They are 15 years old. We just had one of them pass away today. He was my daughter's dog. He was sweet, silly, and had so much energy. He ended up with colon cancer 5 months ago. But XRays recently showed that the cancer was spreading even if he wasn't showing signs of it. We made a decision to put him to sleep while he still acted like himself and was happy.
It broke all of our hearts.
I'm afraid losing him is going to kill his brother. I'm afraid he is going to grieve himself to death. Could this truly be a possibility? We can't go through this another time. Not rght now.