As you said, the difference depends on the relation between sender and receiver. もらうand くれる, we use this words on this situation where the speaker or the person closely related with speaker such as his/her child gets something or sevvice from the sender. And the relation is very important, when the speaker thinks the sender is higher positon , ths speaker use いただく, a little bit higher in the speaker's mind, he/she uses もらう. in the case of equal or a little bit lower, くれる　is used. The important thing is " in his/her mind" What dose the speaker think the relation between two person like? Sometimes the relation dosen't reflect the actual relation. You can get the speaker's thought understandig how to distinguish words, especially novel or manga or TV drama

The focus of くれる is a person who gives something. 彼が（かれが）わたしにリンゴを一つ（ひとつ）くれました。　He gave me an apple. 彼が駅までの道を教えて（かれがえきまでのみちをおしえて）くれました。　He gave me the way to the station. The focus of もらう is the receiver who gets something from someone. わたしは彼から（かれから）リンゴをもらいました。 I got an apple from him. わたしは彼に（かれに）駅までの道を（えきまでのみちを）教えて（おしえて）もらいました。 I was showed the way to the station by him.