Are the Murrys from A Wrinkle in Time Mormon?
No. Wrinkle in Time is a fictional book. Unless the book specifially mentions that the Murrays are Mormon, they aren't.
Confronting IT, Meg realizes that she can free her brother by loving him intensely, because love is an emotion that the evil IT cannot stand. Meg's realization comes through her recalling a Biblical verse that "God has chosen the foolish things of this world to shame the wise and the weak things of the world to shame the strong." Charles Wallace is freed, and the three Murrys and Calvin return home.
Because when they travel via tesseract they are using the fifth dimension which is a wrinkle which lets them travel long distances in a very short time. This basically creates a wrinkle in time hence the title. At the end of the book, the Mrs. Ws create a time wrinkle to cover up the fact that they were gone for many days.