Yáʼátʼééh! In this case, it is not possible to use the possessive “my” for “uncle”, because the prefix a- is indefinite. This exercise is intended to show the difference between the indefinite third person and the singular first person prefixes. Still, I think it is not something that is often used in conversations. And I hope the Navajo learners here can understand that this exercise is aimed only for grammar practice. This is just my opinion about the use of the language program.
The definition of the Navajo word adá’í would be “someone’s maternal uncle”. And the translation of “my maternal uncle” would be shidá’í, using the first person singular prefix. Addressing someone with the corresponding possessive pronoun would be necessary for the Navajo language, on the contrary, addressing someone using the third person indefinite prefix would be very disrespectful. Then, I think the Navajo sentence would not be translated as “my uncle”, or “my maternal uncle”, but just “a maternal uncle”, and “my maternal uncle”, without any relationship between them.
I know many comments are referring to the English sentences, but I honestly think the Navajo grammar also needs to be respected here. There are, indeed, other forms to address one’s relatives without mentioning the possessor that are more polite than in this exercise, but these forms are not showed in the course yet. I hope more advanced skills will be added soon.
Enjoy Walking In Beauty! :)