Sal believes Gramps is taking her along to see where her mother is resting because he knows that Sal wants to see her but is afraid to go. Gramps is going to give her some moral support. In other words, Gramps and Gram are going as an excuse to take Sal to do what she is afraid to do.
Sal believes that her father wants her gone so he can pay more attention to Margaret Cadaver, whom Sal does not like. This is not the real reason, but this is what Sal convinces herself her father thinks. Sal’s father wants Sal to talk to Margaret about her mother, but Sal refuses to do this.
Sal also believes that her father does not trust his parents, Gramps and Gram, on the road by themselves for such a long distance. Therefore, Sal is going along to offer some protection as a voice of reason. The older people will perhaps not take so many chances if their granddaughter is along.