Tâm, in the sentences you provided, "it" is acceptable, because it's not referring to the person directly. Some languages have better ways of distinguishing the topic of a sentence from the grammatical subject, and this is an example of how that lack of clarity in English can be a detriment. If I say, "who's at the door," and you say, "it's my brother," the topic of the conversation isn't you're brother, it's the person at the door, so "it" is fine. However, if I ask, "Where's your brother," and you respond, "It's at the door," that's impolite. Now the topic is no longer the person at the door, the topic being discussed is you're brother. Hope this helps. Another way to look at it is this, instead of saying, "it's my brother," you could be long-winded and say, "The person at the door is my brother." Basically, anytime you might say, "the person...(something)" (in the case, the "person at the door") if it's already know you're talking about "the person...," you can replace it with "it."