I really don't know what to say without sounding shallow. My heart aches for you & your parents.
There's a guy in my car club who was an "in country" Canadian Paratrooper that I think I need to go spend some time with...... I suffered a severe head injury myself about 10 years ago, which has negatively altered large parts of my personality but nothing like what guys like you, your brother & him deal with daily.
Yeah - I need to reach out to him.
Do it. Don't be shy about talking about anything like suicide. Don't tell anyone you think they are going to act but just bring up the subject without making him feel like you're suspicious of his behaviors. Just let him know that IF he ever feels in despair to always, anytime, call. Tell him you love him. Tell him you love him for being willing to lay down his life for your's and others. It will hit him with weight. Believe me.
And for you, a brain injury is a brain injury. A bomb or a car wreck, you're brain takes the hit. If you have brain fog and memory issues it is inflammation. Please look into Millennium. Find an endocrinologist to test your hormones. Inflammation of the brain causes hormone imbalance.