For those who are asking about whether the cuckold is avoidable or not, here is the answer: The mom has a boyfriend named Deon, it's revealed that your mom lied to you about the "not-boyfriend " lame excuse in the kitchen because they already had sex with "sealed the deal" explanation at 9 pm living room scene. If your influence is higher than Deon's, at the same hour and place they will watch porn only.
The older sister has a girlfriend named Unique (for the same BBC fetish reason) and all her scenes happen (your influence on her doesn't affect those scenes) because the dev says she's your ally. So, your relationship with your older sister will be based on triangle love (or corruption).
The younger sister in this version just masturbates while looking at BBC fetish magazine and yes all the family watch and look at BBC fetish content. I think it's the reason that Deon's influence increases the next day.
Resuming, if you don't like any sister's girlfriend scenes, the existence of mom's boyfriend, and the whole BBC fetish theme, I recommend you abandon this game like the developer forsaken Cumming Home 1.0.2 version. If you don't care about Unique, but you don't like Deon's scenes, you just need to keep your influence higher than him. I know there are other reasons like the art style or the story's depth, but I don't think it's the main issue at the moment.