أَبُوكَ — أَخَاكَ — أَخِيكَ
Translation: Your father; your brother (as object); your brother (in a genitive context).
Here أَبُو / أَخَا / أَخِي are each the first term of iḍāfa, and «كَ» is the second term every time.
Key distinction: «كَ» is always in a genitive position, but the first term can be nominative, accusative, or genitive depending on its role in the full sentence.
The genitive status of the second term does not force the first term into a single case.