Isekai Maou to Shoukan Shoujo no Dorei Majutsu - Volume 10 Chapter 5 part6

The old man pointed to the flower-like woman and said, “Call her mother from now on.” He then pointed to a flower-bud-like girl and said, “This is your sister.”

My younger sister was not cute.

She was arrogant, taciturn and weird, but she knew how to please our parents.

She was two-faced and deceitful, but it seemed like she only treated me that way.

I cared for her, tried to please her as much as I can, but she still disliked me.

Until one day, I found out her secret, our relationship began to get a little bit weird…

- Description from Novelupdates

Noah Black, a young man with an abusive father who had no opportunities in life, condemned to live a substandard life because of his father's mistakes.

Noah Black from the parallel universe, a marginalized, obese young man with social problems and a somewhat problematic but loving family, but who couldn't live long because of the mistakes he made.

When Noah 1's mind enters Noah 2's body with a Gamer System, the path to becoming the biggest superstar in this new world begins.

Is that body fat? I work out with the system and in a few days I have the perfect body!

Is my family poor? With my knowledge of my other world, I can make easy money!

Is this face ugly? With System I can look beautiful without plastic surgery!

Do I sing badly? In a few days, the System will transform my voice into the voice of an angel.

Do I act badly? In a few days, the System will make me an Oscar-worthy actor.

I can learn anything I want!

“Rising to the top was like a game, and now I have become the Final Boss of the entire world!” -Noah Black.

Her – a glutton without troubles or worries who crossed over and became a young fat chick with a deep vengefulness.

Him – a young master of the number one power; a two-faced man with elegance and charming, good looks; he is the chosen one of the former saint god and a man who will only bend his back for her alone.

This domineering crossover’s slogan is, ‘Don’t touch if it is this sister’s, put it there if it isn’t this sister’s!’

…watch how the Domineering Glutton and the Two-Faced Venerate Emperor create sparks as they meet and collide, stirring the whole Spirit Treasure Continent into turmoil, how they fall for each other and how they kill each other, how life and death severs love.

- Description from Novelupdates