What type of ending would yall like to see? Personally I'd like a bitter sweet ending where the brother gets to date his sister but his friends are the ones who get to fuck her. More netorase than straight ntr.
You mean her friends?
I actually think now (0.9) is the perfect moment to set up flags for multiple endings.
I like your Idea of an cuck-ending, but you also can easily make a good ending from now on and obviously a bad ending.
Some ideas:
Cuck ending would be somewhat fun, because how pure Seita is, he wants turn 18 before having sex
Good ending, I think, there is enough story build up for that. breaking the curse and leaving the villige, as they planed.
Bad ending, well, it's self explanatory, total whore. ^^