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I don't know what the circumstances are that brought you to the point of living with this regret, but I bet you're the best uncle possible. I've read some of your uncle posts, and I can tell from your writing that you are a good loving and caring uncle. :)

I try to be a good uncle and I think I do a good job at it; I just guess it would have been good to have one of my own. It's ok though, I still have plenty to offer as an uncle.

well, definitely it would have been good if you're considering being something you would have want it for your life. :) Idk if my English helps with what I had in mind but I'm sure you get what I'm trying to say.
I wish for you to spend all the quality time possible with your brother's kids, in the uncle role and treasure all those moments together. These make the real life.....LIFE! (another if you know what I mean).