Some sort of insane meta wrap-around.
I mean "skills" in RPGs are simply supposed to be rough indicators of having practiced things until they're learnt. But it has become so standard in isekai series for them simply to indicate being automatically proficient at things that here the two are shown as entirely separate things that one can gain by separate methods. Like he does have the skills afterall, but just in a typical RPG way rather than an insane isekai way.
Anyway, why is his "older sister" more like a younger sister? I mean she is shorter than him and has a more childish face.