What Coppola did when he cast his daughter Sofia in "Godfather III" doesn't qualify as an act of ego. It comes under the heading of child abuse.
Sofia Coppola, through no fault of her own, other than trying to please her father, was simply in way over her head.
It's hard to understand her father's thinking. In the two previous "Godfathers," his skill as a director seemed to be in guiding each performance, regardless of size, into a shining gem.
For him to not recognize how wrong his daughter was for the part goes above and beyond pure nepotism.