Oh dear, that cliffhanger... hopefully it's nothing too bad.
Personally I wouldn't mind romance between those two. Romance between step siblings is a bit distasteful for some people, but I've seen some rl examples of it so I guess it doesn't hit me as hard when it occurs in manga. Personally though, I think this type of story would be better if the siblings don't get together and instead help each other out with their own romances.