我妹妹 is also correct. Using 的 for very close relationships sounds a bit strange, even if technically correct.
This is accurate. The Duo web notes teach this as well.
EDIT: 我妹妹, is now accepted.
I have also heard a Chinese person simply say 我妹 instead of 我妹妹，I think they should both be accepted.
我妹妹, is now accepted. 11.1.18
"妈妈很喜欢骑马" is accepted in a different problem.
Why is “我的妹妹很喜欢骑马” not acceptable here?