When Yosef took his sons to receive a bracha from his father, Yaakov switched his hands to put his right hand over Ephraim, the youngest brother. When Yosef questioned him about that, Yaakov said that he had switched his hands because the younger brother would become greater than the older one.
The question is: Yaakov had already seen that giving preference to one brother over the other generates conflicts and envy between them. And he also saw how destructive that can be to their relationship. He realized that with his own sons in the story of the Ketonet Pasim and in other situations.
Knowing that, why Yaakov said that Ephraim was going to be greater than Menashe in front of him? Couldn't he tell that to Yosef later on, when the boys would not be around to listen, or just tell them some other reason and keep the truth to himself? Wasn't Yaakov afraid that it would generate some level of envy in Menashe's heart and a fight between them?