Sister problem

Hi, my sister and I have always been really close friends. I moved closer to her a few years ago so that we could do more together and see each other more. I have 2 kids and work full time during the week. She lives alone with her 2 year old German shepherd but works a lot, sometimes long hours. My problem is that it seems like she just wants me to care for her dog as she doesn't like to be home. I pick him up Tuesday after work, have him Wednesday's when I'm working from home and pick him up Friday after work. A few months ago she told me she would like to get another job on the weekend as she needs more money and would like her dog at my house. I specifically told her I couldn't take him on the weekends because I do stuff with my kids on Saturday and clean my house and do laundry on Sunday. Well, she took the job anyway and then cried to me that she didn't have anyone to watch her dog. I finally gave in and said I would take him on Sunday but I'm not giving up my time with my kids on Saturday. I feel like I am getting resentful towards her, it seems like she is taking up all of my time. Am I in the wrong, should I be helping more, I'm just struggling trying to fit everything in and spending time with my kids. It just seems like she keeps pushing her dog on me, is she being selfish. I feel like since he's young, she needs to spend more time with him, please help!