Is it normal for my daughter to be so attention seeking?

My daughter is 3 and usually is the center of my attention. About 3 months ago I started watching my sisters boys, one is 3 and one is 6 months. They've always played together very well (my daughter and nephew) until I started babysitting them all the time. My daughter is usually a very well behaved little girl. Although she is very independent and trys my patience sometimes, I am usually able to correct her behavior. Now, since I have been watching my nephews, her behavior has gotten increasingly worse. With that and the constant fighting between the two 3 year olds. I know to some degree that this behavior is a cry for attention. This behavior was making me lose my patience with her and even made me literally scream at her to leave me alone. Because of this I recently had to make the decision to stop watching my nephews and start spending more one on one time with her. I just would like to know, with all the mothers out there that are raising more than one child, #1.How can I not handle more than one child? #2 is there a way to help my sweet baby girl to adjust to having her cousins around all the time? And by the way since I stopped watching my nephews I have seen a very dramatic positive change in my daughter. I just feel that If my mom raised 4 why cant I handle it?