Possessing My Older Sister

Protagonist lost his parents when he was a child.
The protagonist lived his days in isolation, never experiencing the kindness of human beings.
One day, he succumbed to heat stroke and fainted. Receiving kindness from that episode, he met Aika.
For the protagonist, who lived in solitude, she was like a goddess.
Before he realized, he fell in love with that person and started looking for her.
Finally coming across her, he found her having a loving husband.
Knowing this, the protagonist tried forcibly to make Aika his own...

[Translated from Getchu]