Alright, so I like the idea of a Futa aunt who wants to bang her brother's family, but the structure of the game is highly flawed in my opinion. You make it so that the True Route is to sabotage the aunt and probably eventually seduce the girls yourself as the player. It feels like a game with checkpoints for when you die and I don't think that's fitting for a porn game. Bad Ends Sex Scenes are not good design in my opinion. It's not fun if I want to see a hot scene, only for the game to force me to choose the opposite one right after, making me act as if I wasn't into it
The game should instead follow 3 or 4 different routes with endings dependent on major and minor variables. If Aunty impregnates the girls, then the game should remember that for the endings, which may vary by which girls were impregnated and which weren't. It should also have a way to see how much we sabotaged the aunt or how much we might even have helped.
My personal preference for a route also would be that the aunt isn't an enemy to be stopped, but that we can choose to be her ally and/or love interest, which might require you making the Aunt bisexual. I've seen in some earlier posts that you might consider this for Bad Endings, but that just leads me back to my main point. I want my MC to enjoy being with the Aunt, not to see him possibly get raped and straight after act as if that never even happened.
I realize that such changes would constitute a lot of extra work, but I truly feel that the game would be better off for it.