AITA for refusing to babysit my older sister's kids again?

Recently, my older sister (33), asked me (29)to play with her kids again this weekend. This is not the first time she has asked me for help, and this month, has already refused her several times. She constantly feels that I just have to spend time with her kids since I have “virtually no plans” for the weekend.

I love my nephews, but I have my own personal needs that require attention. When I politely declined this time, she got angry and accused me of being selfish. I started to feel guilty, but at the same time I realize that I too am entitled to my time and personal boundaries.

So who is in the wrong here? AITA for refusing to babysit her kids?