The words above belong to a category of five words called الأسْمَاءُ الْخَمْسَةُ (we will learn the remaining three in later lessons Insha'Allah). When we want to say "Your book" in Arabic we say "كِتَابُكَ" but when we want to say "your father", it would be written as "أَبُوْكَ" and not "أَبُكَ". Similar is the case with the word "brother" when a pronoun is added to it, a letter 'waw' is added as a second last letter and hence the word will become "أَخُوْكَ" instead of "أَخُكَ".

When any of these words annexed with pronouns appear in the nominative case (we will learn more about this later), letter "waw" is added as second last letter i.e., هَذَا أَبُوْكَ or ذََلِكَ أَخُوْكَ but when they appear in genitive case (we will learn more about this later) the letter 'ya' is added instead of 'waw' and the word thus formed is "أَبِيْكَ" or "أَخِيْكَ".