There are three reasons Gramps and Sal's Father give Sal for going on this trip. The answer is found in Chapter Two "The Chickabiddy Starts a Story, Sal List"
1. Gram and Gramps wanted to see Momma, who was resting peacefully in Lewiston, Idaho.
2. Gram and Gramps knew that I wanted to see Momma, but that I was afraid to.
3. Dad wanted to be alone with the red-headed Margaret Cadaver. He had already seen Momma, and he had not taken me.