It seems to me you want your sister to learn how to use credit responsibly. If so, she needs to get some credit, use it and pay the bill every month.
BofA, Wells Fargo, CITI, Capital One all have good secured cards. CITI is a natural since she banks there already. But the others work just as well. Getting a store card is ok, but only if she is a regular customer there, otherwise it is a very bad idea.
Another approach is make her AU on the card you have. Give her the card, tell her to use it and pay it in full every month. You can watch over it, in case there are problems. She said she is responsible, just needs to learn. Credit is something you learn by doing.
I think the AU thing is a great strategy, especially if you just sock drawer the card. I can tell you, without a doubt, that being an AU on an old, high limit Chase card has helped me a thousand times over in building my credit and getting approved for my own credit. It ages my accounts, gives me flexibility with my utilizations and sends a message that someone with good credit trusts me enough to be on their account (I've been told on a manual review that this DOES help, if only a little!). I understand subprime cards and secured cards have their place, but I think you can avoid that for your sister by adding her as an AU and then having her apply for either a lower-end prime card or a store card in a few months.
Add her as an AU. Is she still a student? If so, maybe she could apply for some student cards?
Highly recommended route DCU + Walmart = great builder rebuilders that grows with you that also offers free Fico scores (EQ04 and TU08)save $$$ and know where your at. Then if you really want to help getting an Amex card this year for yourself, if you get a approved add her as an authorized user or (additional card member Amex calls it) to lock in her member start year. This way if she applies for her own card in 5 years, she'll have automatic 5 extra years of history with Amex when she gets her own.
I can only say the same thing as many mentioned already - add her as AU to all your cards.
AU can make miracles happen, sort of
I just always give my example - I've been adding my wife as an AU for all my cards for 1,5 years from the moment I got my first card. She didn't have any history either as we lived abroad. After 1,5 years of such practice she got her first own card - BTW, it was Walmart Store card - and her TU score was 767 at that point, while mine was somewhere around 730
Some time later she's now being instantly approved for cards that I am declined for :-)))
Might also be a wise idea to keep all (or maybe all but one) card by yourself to make sure she can properly manage debt. I'm almost sure that in 12 months she'll be more than okay.