"My sister's toenail."
Translation:Kuku kaki kakakku.
It is an Indonesian tongue twister. The problem is related to the question, which is "My sister's toenail." And I have to explain that in Bahasa Indonesia, when you say "kakak" it actually means an older sibling, period. And you can never tell the gender just by saying "kakak", unless you specify it behind "kakak". This is how you specify the gender: "Kakak" (older sibling) + the gender "perempuan" (female) = "Kakak perempuan" (older sister) or "Kakak" (older sibling) + the gender "laki-laki" (male) = "Kakak laki-laki" (older brother).
If you need any help let me know. My Indonesian is still limited, but much more that what my Duolingo status indicates; however, my English is excellent. My boss, who is a qualified English teacher, comes to me first when proofreading other teachers' report comments. I have taught English in schools, but usually teach maths. From what I have seen so far, you seemed to have a good grasp of English grammar.