"My older sister is good at swimming."
Why am I required to include わたし here? Doesn't あね imply "my" already, just like 母 does, because it's humble?
I think duo just wants you to be specific, but they do flip flop between using 私 and not, which is annoying
こと performs a similar role, changing a verb into a noun, but my teacher explained the difference as こと is used for things that are general knowledge and の is used for things that are specific knowledge to a person
It nominalizes the verb--so it turns "to swim" into a noun, "swimming," that can then be described with じょうず. Leaving off the の would be ungrammatical. (There are some other ways of referring to swimming as a noun, but I don't know if they'd be appropriate here.)
こと works too, I believe の is a bit more casual but there might be other differences.
Duo is fine with me using 泳ぐ（およぐ）in one sentence, but then requires me to use pure ひらがな in another?! Talk about inconsistency! Reported.
Watashi is not required. Especially considering they don’t require it in other questions with other siblings. So why require it here? Reported.