I just thought of a way to resolve this seemingly unwinnable situation for the MC.
He should accuse the goddess/demoness of having an affair with him. This will cause the yandere imouto to find a hax way to kill the goddess, thus preventing any more reincarnations. Actually, this could backfire if the sister somehow steals the goddess's power, which means the MC will never be able to die.
Holy crap, I didn't believe the first warning signs now I regret reading it while I'm on normal state I should have smoke a bit of nicotine before reading this no hate it's just that, it was so scary xD for me well to be honest this is my first manga that i read with a psychological tag.