Should I babysit my sister's five kids in another state for 5 days?

My older sister recently asked me to fly down to her so I could watch her five girls, ages 11 to 5, for 5 days while her and her husband go on a vacation. I have a few issues with this.

One, I work full-time and this trip would require me to take 4 days off of work. I originally requested her to do it over a weekend so I wouldn't miss much work.

Two, I have to get a Real ID so I can fly at the time of the trip. I currently live in another state than where my license is issued and haven't switched over yet. I would have to drive 3 1/2 hours to get back into my home state to get the Real ID process started. In order to do this, I'll have to take another day off of work to do that.

Three, I will have to use almost all of PTO in order to do this trip and still get my regular paycheck. My sister is paying me, but obviously not what my salary pays. If I use all my PTO, I won't have any for the 4th which I usually use to travel home and enjoy time with my family. I also have my mother coming to visit me that same month and planned to take a day off then too.

I have a problem with being confrontational and setting boundaries. I am being guilted by the situation and feel like I'm stuck between a rock and a hard place.

Any advice?