Your Sweetheart is Online (2020)

Both dramas involve a contract marriage between a couple in order for the ML to make his ill grandfather happy,

Both are mini dramas in which the ML enters into a contract relationship with the FL with her pretending to be his long lost sister in order to comfort a sick relative (father vs. grandfather). The two then fall in love.

In both dramas the main couple have a fake brother-sister contract relationship that turns to romance.

FL has a contract relationship with the ML (pretending to be his sister vs. being his wife for a short while) which is to benefit the ML's father in some way. Both are mini dramas.

Both dramas are fake brother-sister relationships between the main couple. And both MLs fall first.
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Both are mini dramas in which the ML and FL agree to a contract relationship as relatives (brother and sister vs. father and daughter) for the ML's need to please a relative, and while living together they fall in love.

Both dramas have the odd concept of the main couple pretending to be brother and sister and living together.

Both dramas have a contract relationship (siblings vs. marriage) between the main couple in order to please the ML's sick grandparent.

Both are mini dramas in which the rich ML is in a contract relationship with the FL to pretend to be someone else (his long lost sister vs. lost fiancée) in order to make his sick grandfather happy.

Both are about a poor FL entering the house of a rich grandfather and his grandson(s) and becoming involved with one of the grandsons.

Both dramas involve the FL pretending to be someone she looks exactly alike which leads to the ML making her pose as that person. She then enters a wealthy family whose grandfather is ill.

Both are mini dramas about a contract marriage that becomes true love. Both MLs marry in order to appease their grandparent.

One is modern and the other historical, but both have the same basic premise of the ML convincing the FL to pretend to be his long lost sister to make a grandparent happy. Along the way, the two fall in love.