I'm honestly glad this isn't yuri, there's enough creepy step-sister stuff on the internet already.
what's wrong with that kind of step-sister yuri ? They only met what ? for a year ? They didn't grow up together, they don't share the same blood, if they met by chance and not through their parents marriage, them becoming a couple wouldn't be an issue, so why would it be just because their parents happened to marry ?
I can understand if their parents got married while they were still very little and they grew up together as sisters, but here, it's not the case.