I think some people are not seeing the ending correctly, they don't have a daughter together, that was his fantasy and his manga that he drew, his alternate future with her that he wanted, but because their siblings they can't have a kid together, so its just them at the end with no daughter. All in all it was a good ending at the end I liked it, you can't help who you fall in love with, so why not just live your life, kinda reminds me of
Saraba, Yoki Hi it has more drama and both mc's are siblings and like each other, but the brother is conflicted with his feelings for his sister, its more of a fleshed out, realistic view of incest between siblings in the real world.