It really feels like six of one half a dozen of the other. Family gets "of me"/mine, and what else please? Animated objects? Feelings? I can't find a good answer. In English.. that's my phone, whose phone is it? It's mine. They're conveying the same message. My mom. My book, my cat, whose is X? It's all mine ,... Wahahahahhha
One of the tiresome aspects of DuoLingo is that it is not consistent in its translations into English. Sometimes, the program insists on sticking close to a literal translation, and other times, it accepts a freer but still accurate translation. Sometimes, DuoLingo even rejects a literal translation in favor of a freer translation.
The general rule is that if there is a literal translation that doesn't sound strange, use it. There is no need to paraphrase. This is the case here: "the apple belongs to me" is perfectly good English, and changing it to "the apple is mine" is unnecessary, since the equivalent to that is different in Hebrew (התפוח שלי).
If the literal translation sounds strange and ungrammatical, usually it is accepted, but a more natural slightly different translation is offered as the main translation. For example:
If you write "I found", it will be correct, but the main translation will be "I've found it".
I realize that your statement is DuoLingo's general policy, but that policy is not consistently followed in every language. Sometimes the students are right.
The upshot is that there seem to be more discussions on why a translation into perfectly good English is considered "wrong" than discussions about real mistakes in foreign language usage.
If you rent an apartment, you might find yourself explaining to someone "yes, this house is mine, but it doesn't belong to me". It's a nuance, and it's yet harder to make this distinction with apples - but since both Hebrew and English has exact parallels for this difference, and both forms are very useful, it makes sense to insist on learning both.