あなたは私（わたし）にとって兄弟（きょうだい）みたいに思って（おもって）いるよ。 The direct translation would be... "I am thinking of you like a brother to me." Probably too much like a computer translation..... Let's say あなたの事（こと）は兄弟の様（よう）に思ってるよ。 That would be the sentence you want to use. However, Japanese people might use 兄 (あに: elder brother) or 弟(おとうと: younger brother) instead of just brother（兄弟）. Same for a sister. The word for a sister would be 姉妹 (しまい). However, the word 姉 (あね: elder sister) or 妹 (いもうと: younger sister) might be better. You can put either of those words instead of "brother" in the sentences above. This is just a culture with no particular logical reasons, I guess. We are used to say in this way. Either would work. Enjoy!