Which was the most stupid thing Father Manuel ever did?

I think there should be no doubts - he told Millie that Federico is her father without telling her that Federico isn't Ivo's father. Why??? The only reason I can think of is that this was the only way the authors of MB could make Ivo marry Florencia. (A really stupid reason, if you ask me...) But if we put aside this reason, Father Manuel had completely no reasons to do that. He knew Millie was in love with Ivo and he knew Millie would think Ivo is her brother and he knew that actually Ivo isn't her brother, so why didn't he tell her that???