So, after an old story japanese story named The Tale of Genji, where a man named Hikaru Genji who kidnapped a poor girl that lived in poverty and raised her with the plan of marrying her when she grew up, stories featuring an adult raising or helping raise a non-blood-related child and later entering a romantic relationship when the child is grown up became known as "Hikari Genji plan".
One infamous case that I will not name is a manga where the first half, that if I recall is the part that became an anime, was a loved slice of life but after a timeskip the second half became this. Let's just say that, since most fans joined for the cute family of the first half, the second half is... not as liked (I heard it bein called a betrayal, even).
According to Tv Tropes it used to be pretty common in older works because it was common for noblemen to help raise the daughter of friends and later in life they may sometimes fall in love and marry each other.
I heard that it was mainly to avoid this that there was a law in Japan that would make it ilegal to marry someone adopted into the family or that joined it through marriages, for example stepsiblings like in this story's case.
But I also heard later that the law was repealed because of the low birth rates in Japan.
