"我阿姨" should also be right.
if you click on family lessons you get at the bottom a small summary that for pronouns you don't need to use 'de'. Normally I would have used 'de' but because of that summary I skipped it and bam 'error'
"我阿姨很聪明，她是老师。" was marked wrong. So are we trying to say the aunt isn't a close enough family member to warrant dropping the 的
Is that how we differentiate when we can/cannot drop the de? If so that's quite a little tip!
I suspect that whether you drop the 的 or not depends on the level of formality you're going for.
This exact sentence as you wrote it appears in another exercise of this lesson.
No, both with and without 的 are correct (with 的 is slightly more formal). This is in beta and it takes some time to make all the corrections.
Unfortunately no longer in beta, and it's still not corrected.
Acutally, "aunt" as family member is called 姨妈, and 阿姨 is a friendly expression when calling a woman(except for young or single).