In the second half of the game, win as a man but you need certain stats to dominate your sister and not Ella. I don't know the exact requirements but this is how I got it.
Don't flirt with Ella in the first half of the game, and in the second half of the game be a girl and lose the therapy quiz against your sister. Reject becoming a sissy/bimbo and go back in time to the second half of the game, this time as a man, win, and instead of dominating Ella you dominate your sister. You should get something like this: