One common theme could be the development of their relationship. Since it's fanfiction, it might explore different aspects of how they get closer to each other.
I'm not sure about specific 'keita and ako lemon fanfiction' as it might be quite niche. You could try searching on fanfiction - specific websites like Archive of Our Own. There might be some works there related to your request.
Love and family are common themes. Since they are in a rather unique family situation, many fanfictions explore how they navigate their love for each other within the family context.
One great fanfiction could be 'Ako and Keita's School Days'. It focuses on their daily adventures at school, including how they handle the unique situation of their relationship in a school setting. There are also some really sweet moments between them that are well - written.
Riko in fanfiction usually has a lot of energy and enthusiasm. She could be the driving force behind many of their joint activities. Ako is often characterized by her kindness and thoughtfulness. She might be the one who remembers important dates or takes care of small details in their lives. They both, however, share a deep sisterly love which is a common theme in most fanfictions about them.
Typical themes might include the relationship development between Riko and Keita. It could involve their daily interactions, how they handle the unique situation of their relationship in the context of the 'kiss x sis' story.
You can try fanfiction.net. It has a large collection of fanfictions from various fandoms, and there are likely to be some good ones for Keita x Kazuki from Gakuen Heaven there.
One good story could be 'The Bond of Keita and Kazuki'. It focuses on their growing relationship within the school setting. They face challenges together and their friendship slowly blossoms into something more.
One common theme is friendship. Since they are often characters in the same story or universe, fanfic writers like to explore their relationship as friends, how they support each other through difficult times.