MTL - Shoujo Grand Summoning - ~ 041 tells the existence of the sister-in-law...

Mo Yan was an illegitimate child of a rich family and was arranged by her mother to take the place of her sister in marriage to a poor man in order to fulfill the marriage contract set by the previous generation, allowing the Mo family to get their hands on that substantial dowry.

[Congratulations! You’ve unlocked the Dimensional Motel System.][First Mission: A runaway princess—hailed as the most beautiful in the land is being hunted by her step-mother. She was originally headed to the dwarves’ cottage, but the System has rerouted her to your motel instead.][Protect her. Win her trust. And if you manage to woe her, you’ll earn her Affection Points.][Bonus Mission: Sleep with her evil stepmother to unlock more reward]'What kind of weird mission is this? And why does this story sound way too familiar?'

“I can’t do it anymore. Stop it…I can’t take it anymore!” The second young master stopped in his tracks and raised his brows as he stared at Miss Jiang. “All you had to do was jog, so—”