The Nineteenth Year

Both couples grew up together since childhood as adopted siblings with the ML falling first and becoming both possessive and protective of the FL.

Both are short length dramas about a star-crossed romance between a pair of adopted siblings who fall in love as adults.

Both have a protective relationship between the leads that begins as adoptive siblings and then evolves into a romantic relationship with the ML falling first. Both have a SML who has a one-sided love for the FL but respects her choice and helps the couple.

Both leads are raised as siblings, ML loved FL from long ago but is a little mean to her.
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This movie is also about love between so-called stepsiblings with no blood relationship. The ML also seems cold-hearted at the begining.

They both focus on the romantic relationship between step siblings and are vertically filmed micro series.