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

Zaiyila thought she had suffered enough—betrayed by her own mother and sold into marriage with a wealthy, aging man. But on the night of her pre-wedding ceremony, everything takes a dark and unexpected turn. A swarm of deadly snakes descends upon the gathering, killing her groom-to-be and throwing the hall into chaos. Fear spreads as the people recognize the serpents as the servants of the enigmatic and terrifying Snake King, Everett.When Everett himself arrives—a striking, horned man of unmatched beauty and power—Zaiyila finds herself drawn to him in ways she cannot explain. Despite the fear he inspires in others, she feels an unshakable connection to him, one that transcends the realm of logic. The moment she defies the fearful kneeling of the crowd, she unknowingly seals her fate.Taken into Everett’s domain, Zaiyila is forced to confront not only the dangerous, supernatural world she’s been thrust into but also the awakening desires she cannot deny. As she unravels the truth about her past and her connection to the Snake King, she must decide—will she resist the pull of destiny, or will she embrace the darkness and passion that Everett offers?A tale of betrayal, power, and an irresistible bond, Bride of the Serpent King explores the fine line between fear and desire, fate and choice.---

A world only I know of, now I will take it all for myself.There’s no kind soul here willing to share and eat nicely with others.Only the ruthless one who will devour it all alone exists here.

Shen Yinning was originally the Crown Prince Lu Ying's childhood sweetheart and fiancée, but due to her father and brother's rebellion, she was exiled to Gan State and married to a minor local official as the daughter of a criminal. Meanwhile, Lu Ying gloriously married her cousin Shen Yunxi as his Crown Princess Consort.Three years later, Shen Yinning returned to the Capital with her husband. Everyone praised the Crown Prince and Princess for their mutual respect and harmony, and Shen Yunxi often flaunted in front of her how loving she and the Crown Prince were. However, only Shen Yinning knew that the seemingly restrained and honorable Lu Ying came to her chamber every night, completely disregarding the fact that she was now someone else's wife, secretly claiming her and keeping her as his Forbidden Fruit...

I am Master Siwei, and I speak for arcane arts.
This is an era of heroes and heroines.
This is an era of great changes.
In the end, it will be an arcane era opened by me.

- Description from Novelupdates