Both sentences sound a little weird in terms of logic to me. The first should be 兄は泣いたことは一度もありません。Using the particle "mo" followed by a negative makes this sentence mean "my brother has never cried, not even once." If you don't want to use the negative ending, you can say "兄は泣いたことは一度だけあります” to mean "my brother cried ONLY once." The second sentence makes it sound like your older brother is a cry baby, and he *always* cries. However, there was at least one occasion he *didn't* cry...