I was much younger than you were when my brother made me much the same promise--a promise to look after me and that I would never lose him. Even at that young age, I made him the same promise. And in the Petrelli family, the rule about promises is very simple: a Petrelli makes a promise, a Petrelli keeps a promise.
Times changed, as did my brother and I, and we were not able to keep those promises we made to each other as much as we would have liked to. It is only in recent months that we have...reaffirmed those promises we made decades ago. And so far, we're both doing a very good job of keeping it.
Your desire to make a promise to him that you can keep is admirable. And I wish you only the best in reconciling with him.