Who makes the final decision about taking along Casy, and what does this decision reveal about the power hierarchy in the Joad family?

Ma makes the decision, and she seems to be pretty sure the men won't argue the fact when they have time to talk. She defers to her husband when she must (or in this case her son Tom), but her opinions and desires are an important part of the power hierarchy within the family.

"Ma looked to Tom to speak, because he was a man, but Tom did not speak. She let him have the chance that was his right, and then she said, "Why, we'd be proud to have you. 'Course I can't say right now; Pa says all the men'll talk tonight and figger when we gonna start. I guess maybe we better not say till all the men come. John an' Pa an' Noah an' Tom an' Grampa an' Al an' Connie, they're gonna figger soon's they get back. But if they's room I'm pretty sure we'll be proud to have ya."