The answer to this question is actually only mentioned very briefly in this excellent story and comes towards the end when Mama refuses to give the quilts to Wangero, otherwise known as Dee. The idea of Maggie marrying anybody seems rather slim, especially because of the animal imagery that is associated with her character as she "cowers" away from the other more confident characters in the story. However, note what Mama says when she says to Dee why she can't have the quilts she has chosen:
"The truth is," I said, "I promised to give them quilts to Maggie, for when she married John Thomas."
We are never told more about who John Thomas is or when they are planning to marry, but this is the man that the story tells us Maggie is going to marry.