1) 우리 형이 집에 도착하면 나한테 전화해. 이 just marks the subject of the sub-clause. 우리 형 is not the topic of the sentence, but just a component of the condition 우리 형이 집에 도착하면 (if/when my brother arrives home). The structure is [우리 형이 집에 도착하면] 나한테 전화해, where the bracketed part is a conditional sub-clause. 2) 우리 형은 집에 도착하면 나한테 전화해. 은 makes 우리 형 the topic of the sentence. You are giving some information about 우리 형 that you know and feel about in a certain way. The sentence structure is like 우리 형은 {집에 도착하면} [나한테 전화해] = Speaking of my brother, [he calls me] {when he gets home}. Put in simple terms, 은/는 starts a sentence that originates from the speaker's mind - the speaker has the knowledge and feelings about the subject and expresses them. 이/가 just indicates something as the subject, not necessarily the main interest. If you saw a traffic accident, you'll talk about it with 이/가 (길에 교통 사고가 났더라) because you're just describing an external thing. If you want to talk about what you know or feel about accidents, you would use 은/는 (교통 사고는 대다수가 운전자의 부주의 때문에 일어납니다). What I said above is a little simplified, but it still describes the most fundamental difference between 은/는 and 이/가.

Generally, in the complex sentence, the topic marker 은/는 is used for the subject of main clause and the subject marker 이/가 is used for the subject of subordinate clause. If the topic marker 은/는 is used for the subordinate clause and the subject of main clause is omitted, the he topic marker 은/는 is also applied for the main clause. 1) 우리 형이 집에 도착하면 나한테 전화해. = 우리 형이 집에 도착하면 너는 나한테 전화해 우리 형이 집에 도착하면 is subordinate clause (너는) 나한테 전화해. is main clause 2) 우리 형은 집에 도착하면 나한테 전화해. = 우리 형이 집에 도착하면 형은 나한테 전화해 우리 형은 집에 도착하면 is subordinate clause (형은) 나한테 전화해 is main clause

Thank you all for the replies.

When it comes to Korean, context is everything. BUT 은/는 and 이/가 has diffferent nuances accosiated with them, but it's quite hard to translate it to English, as there isn't such concepts in English.