It should have specified "older sister"
Right? Since it always corrects me if I'm not 100% specific.
Picture didn't give しゃしんas a hint
It does, in kanji
Look, if any of your questions say just "sister", then いもうと, あね, or even シスター can be correct. Specify if you want me to.
妹 (いもうと) still not accepted (November 11th, 2017)
Why is it 見せます and not 見ます?
Because that would be read as 「みます」meaning "to see".
姉に写真を見せます。reported on 6/13/2018
姉に写真を見せます。is not accepted as of 1/10/2018. This has been reported.
Especially given that 写真 is the hover-tip given by Duo, I expect it to be in the accepted answer.
Why does the "my sister" NP have to come first?
I don't think it's technically incorrect, but would certainly sound awkward.
Why is this sentence wrong: 私の姉に写真を見せます。