Honestly, seeing people freak out about the "incest reveal" this chapter is kind of funny. This is just so obviously tame and it's obviously not going to go anywhere. Plus, I've read and finished things that were much worse in the incest-department— So it's a bit hard to get upset about something like this.
Like, compared to a manga that goes 90% of it's run with the sister being basically just a mob character to suddenly having her become a super-incest exhibitionist and winning girl with absolutely no build up or justification, (And no incest tag!) this might as well be nothing.
And she'll probably die at some point to give him a power boost, anyways.