Welcome and bless you for being there for this little one who had such a rough start in life. As cruel as some can be, I also like to look at the other side -- there are a couple of really good people in this little one's life already -- you and your sister, the vet!
I don't know how some can be so mean to defenseless little animals. There are no excuses for them. But we can only concentrate on trying to do right by them ourselves. Animals so often deserve better than they get. That is how I justify spoiling mine -- bringing some measure of balance.
I don't feel like I should be giving advice on house-training as I am not making monumental strides with my little one. Just as we had made some headway, we had this cold spell and she does not want to do anything when we go out now. She just whines to go back in. Poor little baby. I put a sweater on her too!