The sister is in charge of the program the MC is in so she is technically his boss.
She isn't dominant to him in a sexual way, there is no incest. It's not the devs fault you can't see beyond sister fucking.
Some people have a need to make every game for them even if it very clearly isn't.
Go to any thread with a fetish some people won't like, gay, lesbian, NTR, hotwifing, femdom, female protag etc...
You will always see at least a handful of people asking for entirely different content or for content to be changed to suit them. For instant female protag games have a LOT of people demanding the dev either change it to male or add a male protag. Femdom gets a lot of people asking for ways to reverse to maledom. Lesbian gets a LOT of people asking for straight content. NTR gets a shit ton of people raging against it and the dev.
If a game doesn't have incest then you get a ton of people demanding incest.
If you are a dev and you try and cater to a specific kink or group you will always find people trying to change that or get things added to suit them.
Some people can't take no for an answer and can't seem to be able to just move along.
Sadly a lot of devs cave in and give in to their demands which makes it harder for devs that don't as those people then expect it from all of them.