"I show a picture to my sister."
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.
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.
I don't think it's technically incorrect, but would certainly sound awkward.