I wonder if you are mixing up multiple stories here. I remember vaguely a couple where the main character asks to be forgotten, so no-one will mourn him.
But the second part sound very much like "Seichou Cheat de Nandemo Dekiru you ni Natta ga, Mushoku dake wa Yamerarenai you desu" a.ka. Growth Cheat. The main character gets accidentally blessed by two goddesses, giving him 400x XP growth. And he learns he can meet the goddesses again by clearing dungeons. When he does, he asks the first goddess if he sister is okay. The goddess promises to check, but as soon as the MC is gone, she notes that he isn't supposed to have a sister, and starts to investigate.
It turns out his sister is the reincarnation of the demon lord from the isekai world, and she later arranges to get returned to that world. But she does so on her own, not with help from the goddesses.
Also, while the MC of Growth Cheat does not request to be erased, we do find out that he has disappeared from photographs and his sister is the only one who remembers him, for the reason spoiled above.