When I thought about answering this question, the women, in my own life (wife, relatives & friends) came to mind. For the women I know, the choice would have to be a 20 gauge over the 12. They just wouldn't practice with the 12 gauge. The 20 gauge would be ideal for most of the women I know. I agree with those who stress the importance of good training and practice for your sister. If she is serious and committed, it won't take long for her to become proficient. In 20 gauge, I like the Mossberg Tactical 6. You should also be able to find a used side by side for a reasonable price. Stoeger makes an economy level side by side and over/under in 20 gauge that would likely get the job done for her needs. I would avoid the single shot, although one is definitely better than none.
http://www.budsgunshop.com/catalog/p...+Cylinder+Bore