Our mc lacks suspiciouness. When his sister reincarned as his maid (what a coincidance), he should obtain CautiosHero syndrome. I hope he will get gooder soon.
Our MC is a careless idiot. That's my diagnosis. That crazy sister can reincarnate as every female slightly younger then him so he should be always alarmed meeting them. Cautious Hero would be disappointed.
Ok. From this moment MC's objective is: Don't let her kidnap you. This shouldn't be too hard especially when she's younger and weaker. (I mean, after next reincarnation). Don't get poisoned. Ez