No, you are not selfish. i think you should be firm and tell her this is time for you to spend with your daughter, and suggest maybe she should do the same with her child, but just not with you. I know how you feel, sisters can be so overwhelming sometimes and have a tendency to make you feel bad. We love them dearly, but we also have a life without them. In your case put your child first and if she wants her child to have a holiday, then that's her responsibility to book one without you.
My sister asked me to take her daughter on my family holiday, but i said no. Reason being, this was my first holiday with my children, and i wanted to spend quality time with them, without anyone else around. I don't see that as being selfish. Selfish is parents wanting to dump their kids on others! enjoy your holiday hon xx