I absolutely feel your pain! I introduced Hallie (who was 2) to Simon (who was one) and they did not get along at all either at first. Simon was pee pad trained and Hallie was not. I struggled to get them familiar with each other and also with the potty training which I started over with Hallie since I wanted them both to be consistent. Needless to say it took a lot of patience and a lot of time but it does happen. How long have you had her? It took about 3 weeks for Simon and Hallie to "bond" and during that time I had to give them individual time with me to make Hallie feel comfortable and to make sure Simon got enough attention. I don't know if this is helpful but I hope it is encouraging, I know your frustration but don't give up. It will happen
Kim, Simon and Hallie