It's quite annoying that Duolingo marks it wrong every time I omit "的" in sentences like this denoting the relationship between family members. I've reported quite a few sentences now.
Yep, same issue. They are way too rigid in both the Chinese and English grammar that they will accept, which I understand takes time but the 我/我的 issue should have been anticipated.
February 7, 2019: I entered my answer without 的 before 妹妹 and it was accepted.
Just don't translate little sister as 小妹妹 :-)
小籠包 doesn't have to be just pork
Yes, but by default it is.