Dear 4_7,
Why do you doubt the priest when he says “No a Mormon can not be a godparent in a Catholic Ceremony”. It is “NO”.
The Mormon religion is “totally” different from Catholic teachings.

If you have your brother as a godparent, then that’s sufficient. The purpose of a godparent is to assist the parents of the child with the child’s religious education, i.e. Sacrament of Reconciliation, First Communion, etc

So, check out with your Pastor if one person is sufficient for a God parent.
Best Wishes,