Sure, some people don’t mind this kind of relationship (please, seek professional help), but he raised her from when she was six years old. That’s essentially like having romantic feelings for your father. Think about that. Even if they aren't biologically related, he still took care of her from childhood. In away the ending seems kind of abrupt, as well as it wasn't. Yes they kind of hinted it earlier from Rin's part, but I didn't expect Daikichi to entertain it.
All I can say is that I recommend watching the anime instead, since it does not follow the manga’s ending. If you want to still read the manga but don't like the ending that I just mentioned stop around chapter 24.
