Negai o Kanaete Moraou to Akuma o Shōkan Shitakedo, Kawaikattanode Kekkonshimashita ~ Akuma no Nīdzu

From the first two chapters of the manga I have deduced:

Orphaned shota adventurer tries summoning the devil to help his career as his own skills are lacking. Surprising everyone, the devil is a busty onee-san who offers to grant him one wish. He’s so enamored with her appearance, and somehow had no clue how strong and evil a devil really is, that he asks her to be his wife instead of anything related to his adventurer career.

She agrees and they both turn out to be pure and naive as each other when it comes to romance.