A two-year-old can handle two days with Dad instead of with Mom. If you want a corresponding guilt to push you, then remember that you have a relationship with your sister that deserves your time and effort, too. Don’t take her for granted! If you fast-forward and ask your now-twenty-two year old child, he is going to look at you as if you were nuts. If he even remembers those two days, he won’t think you should’ve stayed home to spare him time with his father!
There: Two days of your sister’s needs, and your child’s needs will be seen to by that other parent he has.
PS Children have been known to get in a little huff when Mom comes back. Be ready for this, and do not take this personally. They get over it. You should, too.