Responses
I believe it is this one:
https://www.mangago.me/read-manga/dear_sister_i_ve_become_a_blessed_maiden_uncensored_ver_official/
beta
The plot revolves around a brother and his sister, who suddenly get transported to another world. They are summoned by a mage, who is an extraordinary asshole. The mage explains that he planned on summoning this younger sister only, to make her his bride. Older brother hates that, and instead offers himself up. Mage asshole says haha okay, and sends the sister back.
Mage explains that MC (through a bond) can supply energy for the mage. Through intercourse, obvi.
I'd appreciate the help if someone could find the title!! ╮(￣▽￣)╭