"Is eating" sounds more natural in English for something that is being done right now, rather than a repeated occurrence. If she's eating one apple today, then another tomorrow, and another the day after that, then she eats my apples. If you're just saying that she's eating one right now, using "eats" is grammatically correct, but normally you would say she is eating my apple.
I'd guess "Sis" as a translation of сестра wasn't accepted. You could report that, but I would recommend translating as sister in future.
This is simply incorrect. I wrote: "A sister is eating my apple" and it was marked wrong and should be "The sister is eating my apple.". I'm just reviewing basics, and I'm telling you ... it's not right to mark that as wrong. "A sister" is just as acceptable as "The Sister". Doesn't matter how it could be used. With how we have to insert the indefinite article in English for the lack thereof in Russian? Both should be correct.
@ericashelton - Because there's no article or anything, that's definitely a legit translation, but "a sister" in English sounds like a female member of a religious order (like a nun at a convent).
With such limited context, both "a" and "the" sound kind of weird to me though :\
It is because яблоко is a neuter word.
мои is a plural.
мой [moy] is masculine.
моя [moya] is feminine.
моё [moyo] is neuter.
Credit to garpike and ILoveHens ( https://forum.duolingo.com/comment/15030386/What-is-the-difference-between-%D0%BC%D0%BE%D0%B9-%D0%BC%D0%BE%D1%8F-and-%D0%BC%D0%BE%D1%91 )
The past tense of yest is yel/yela/yelo - that would be imperfective (she was eating my apple). The perfective would be "съесть". Past tense would be Съел/Съелa/Съелo which would mean she ate the apple and the action is done. Sorry for switching from Latin to Cyrillic there...