Well the Cover pages stated multiple times that it is a family romance story. Therefore, the romance was to be expected. I would have liked it without the love story as well but they fit each other quite well.
Other than that i dont mind them being step sibblings at all. It would be very strange if they grew up together. But they have been a family for what, like 3 months? You cant just suddenly switch off attraction just because the parents got married. I honestly think its very normal that they'd develop feelings for eachother after spending that much time together. Also ive heard from a lot of friends whose parents remarried (the friends were all over 13 when the marriage happened) they sooner or later had a crush on a step sibbling. But they all didnt do anything because it would have been awkward for the family and hella complicated.