Haikei, Tengoku no Neesan, Yuusha ni Natta Mei ga Erosugite: Ojisan, Hogosha toka Sorosoro Muri desu+

Angel, a former top-tier mercenary, was forced to retire due to an injury. Left by his fiancée, he now spends his days drowning in alcohol—a middle-aged adventurer (and a virgin). One day, his niece Liz, whom he once taught swordsmanship, suddenly barges into his life! Angel is bewildered by how she has grown into an astonishingly beautiful girl, but Liz pays no heed to his hesitation and boldly declares that she will become his wife...!? A heart-pounding fantasy adventure, in more ways than one, featuring a beautiful young heroine and her middle-aged uncle!

(Source: GOT Corporation, translated)