The webpage where you sign is is both confusing and jumpy (meaning page elements move abruptly when you click on anything, which suggest poor coding--though I noticed tonight it's not as bad). You need to click in the upper right corner ("Sing In/Join"). That takes you to the Sing In/Join page. If you have joined and used the same browser, your username and password (covered by dots) will be displayed, so you just need to click on the Sign In button/bar. If you are using a new or different computer (like your sister's) you will need to type in your username and password before clicking Sign In. Coke would love to have you sign in through Facebook or Twitter but you don't need to. I don't.

I was asked to give an answer, but I have no idea. Maybe you could ask the Coke people directly at their website. Sorry I'm not of more help.