usually when I see people complain about genre issues it seems to be out of complete lack of understanding the genre.
I wouldn't be as affirmative as you.
Whatever if it's adult content or not, there way more space for changes than you make it look. It's clear that all genre have their codes, and that breaking out of them isn't something easy to do, but it also doesn't mean that you have to include everything wrote in the said code.
There's incest stories where the mother, or the sister it depend, is present, but not among the characters you can pursue. There's incest stories where the relation started before the story, and therefore everyone is cool with it. There's even many, mostly on the shota side, where the MC is forced into incest by the girls in the family.
The cliché you named don't exist because it's parts of the code, but because it's what
Sisterly Lust used as setting. It's the game that defined the trend, with a MC coming back home, the prudish sister, the bitchy one with her broken heart due to MC disappearance, and the madly in love one. But its not a trend because incest games need to be like this. No, it's a trend because the game was a success and you'll always find people to copy what is successful in hope to drown in money. Then with the time going, there were enough games following those steps for some new creators to believe that an incest game must be following this setting.
But in the same time you can also found many other incest games that don't looks at all like this.
My New Memories and
Bad Memories are part of them and not less successful.
The issue isn't that a genre need to follow a code, but that too many creator don't have enough imagination to starts a new interpretation of the codes. Or perhaps it's not even a question of imagination, and just that being amateurs they feel more at ease by following a signposted path. This way they can compare themselves to something and know if they are doing bad, average or good enough.