My Big Sister Lives in a Fantasy World: Humanity’s Extinction Happens During Summer Vacation

Yuichi, and the survival club are finally heading off for their summer training camp, to a luxurious villa in a remote seaside resort. Will anime-style swimsuit episode shenanigans ensue? If his big sister has anything to say about it, yes!
But while the gang enjoys some much needed fun in the sun, a darker plot lurks just offshore. The world's strongest little brother is about to face his greatest challenge yet!