Marmalade Boy, Honoka ni Purple Recommendations
There are quite a few similarities between these two. In both, there are two families where the parents remarry. Then of course the non-blood related, newly formed siblings fall in love with eachother! Both also have important scenes in libraries... and some suprising plot twists! Both are really cute and fun to read too!!
In both mangas the main characters suddenly become brother and sister, both stories are very romantic and fun to read.
Have a similar plot. The two main characters are step siblings that are not related by blood. They tries to be together but their father is getting in their way.