"He demanded that I inform him about everything."
Translation:Li postulis, ke mi informu lin pri ĉio.
The word "sin" is reflexive; the subject of the clause (after "that") has to be performing the verb on itself (and is generally restricted to third person / indefinite pronouns). In this case, "I" and "him" are different people, so you cannot use the reflexive like that. If the sentence were "I demanded that he inform himself about everything," then using "sin" for "himself" would be correct.