"My sister is taking a shower."

Translation: あねはシャワーをあびています。

[EvanHerrin3]

Is this the title of a light novel?

[ivanpopjovanov]

Nah, it's too short for a ln title

[KaterinaRuud]

When do I use あびます and あびています?

[KagayakuSeiza]

The ています ending indicates continuous action.

Basically:

あびます=takes a shower

あびています=is taking a shower

[V2Blast]

It marked me wrong for translating "sister" as 妹（いもうと） instead of 姉（あね）, even though the English sentence didn't specify whether the sister was older or younger. I've reported it (Jan. 5, 2018).

[YAMAyuly]

姉(あね)/妹(いもうと)　は/
が　シャワー　を
浴びて(あびて)　います。