"My younger sister enjoys horse riding."

Translation: 我的妹妹喜欢骑马。

[emperorchiao]

我妹妹 is also correct. Using 的 for very close relationships sounds a bit strange, even if technically correct.

[LazyEinstein]

This is accurate. The Duo web notes teach this as well.
EDIT: 我妹妹, is now accepted.

[Phillbo]

I have also heard a Chinese person simply say 我妹 instead of 我妹妹，I think they should both be accepted.

[Celticfiddleguy]

我妹妹, is now accepted. 11.1.18

[ShaneNT]

"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?