Honestly, I don't see where the problem is with them having a relationship even tho they're step-siblings considering that they are teenagers that were complete strangers to each other before starting to live together, like, it sounds reasonable to me that they could catch feelings for each other in that situation.
Like, for the people that are calling this incest, imo it can only be called incest when at least one of those two things is true:
- They're blood related siblings (even tho if there's an extreme situation like "they've been separated at birth and when they're older they find each other not knowing that and catch feelings for each other" it would still be fine as long as they don't have kids, but that's so extremely unlikely to happen it's almost useless to talk about);
- They lived the whole first part of their life as normal brother and sister, so not like a situation like this manga where they started living like that when they already were teenagers.
Other than those I don't see anything else as being incest, I might be wrong, but it just makes sense to me like this