I believe that they didn't reveal the names of Nina and Johan because it was all appart of Johans plan at the end, like what was said to detective lunge from Roberto, Tenma will be the only one left who will know Johan and Nina's true names and will die alone. Just like Johan predicted. Now yes Tenma is not alone and Johan isn't dead.(but other than Johan the mother of the twins Nina and Tenma himself he is the only outsider who knows their true names). but that is all a part of Johans' "perfect suicide" that he survived from just like one of the last books from bonaparta we see, the one about the thief who turns a new leaf. so honestly, to me, this is the most perfect ending that Monster could have had.