Incest is the best part in these stories!

About your question; According to comments on novelupdates;
In volume 7 of the novel mc is ready to accept sister's love & have plans to marry her in the future. And no, there's no copout like them being step-siblings.
...but, I don't think that's what you really should worry about;
Apparently, according to comments on novelupdates, in volume 8 he's ready to accept his brother's love. And apparently there's a witch that can make men pregnant. Go figure. If the manga ever gets to that point, I drop it. Or probably sooner...