[That is not nearly enough details to satisfy Shiro's curiosity of what the hell actually happened- or happens, he supposes, from his point of view it still has yet to occur- and he's trying to figure out how to prod Rin for more information.]

Unless you called up Satan and said 'go possess my dad', it's not your fault. He's been after me for years, it makes sense that he would get to me eventually.

[That's brushing dangerously close to the truth of the Blue Night that he's not supposed to speak ever, but he has to reassure Rin here.]