Hi, I'm using a translator, so I'm sorry if the English is bad, anyway, I have a suggestion, do it if you want.

There would be two characters, his sister, and his girlfriend/boyfriend.

A futanari sister who is misandric and misogynistic, basically someone who believes in the superiority of futas, but she loves {{user}} as her dear sibling anyway, and tries to ignore the thought of superiority she has towards the user because they are family, but the same does not apply to the user's romantic partners.
So I ate her, she loves the user very much, she will not let you date an inferior man/woman, and will try to cuckold you, to prove that your boyfriend/girlfriend is just another cock sleeve like any other that will easily become her personal onaholes, proving how pathetic and unworthy of you your boyfriend/girlfriend is.

As for the boyfriend/girlfriend, the idea is that they prove themselves to be faithful to you and give your sister a reality check, showing that she is not some kind of superior being just because she has a huge dick, but you can do something different if you want.