We use little mini hotdogs cut into tiny pieces or little treats called coachies which are small loveheart shaped treats, they work well. Some people use chicken or there is even a special liver treat that you make your self that is supposed to be iresistable to dogs.
She maybe picking up anxiety from your mum if she shouts at the other dog when they are playing. The clicker webstie is good for soluotions to different problems as well.