Sharing sister's account at different home

Does she have DirecTV satellite service or one of the streaming-only services? The Gemini (C71KW) is available for both (different firmware versions), so the answers depend on which service it is from.

And is that box from her or somewhere else?

She has the Directv Streaming service bought from AT&T. This is the STB I bought Pre-Owned AT&T TV FHD OTT C71KW-400 Client Streaming Box   I thought 
i could get their free service to try before I got a paid plan but NO it will not work on that device, ONLY paid services  

I received am email from DIRECTV Community Forums that

 "  Saind commented on a post you are following." It did not show up on my thread.

Unless it was deleted I would like to see the post.

Looks like that user is blocked and had a string of posts. So appears to be a spammer/scammer that got kicked out.

Since you got a pre-owned box, hopefully the prior owner removed their account info. Also if it had AT&T branding the concern is when it was last used. They updated the web address for the required updates. If the box wasn't connected in that time frame and did not get the update on where it would get future updates there is no way to do that manually.

When I set it up It said it was up to date. I had to put my info in so I think the prior owner removed their account info.