I Think My Brother-In-Law is Crazy....Do You Agree with Me??

My brother-in-law John has two children from his previous marriage. He has also 2 boys with my younger sister. John has never told my teen nephews about their half siblings. Recently and mistakenly, I mentioned the truth to my nephews about their other half siblings. John found out that I said the truth and he was so upset as hell. He screamed and shouted at me and quit talking to me. He said he was not ready to tell my nephews the truth yet. He was afraid that this finding will create some kind of resentment between all siblings. Is he correct with his feelings or is he just plain crazy? I'm just curious to see how most people feel about this issue...Thanks for your input.