Q. 1. My sister, a married adult, is getting baptised next month. She would like her husband to become her Godfather. Is that allowed? I cannot find any information on this matter.
A. 1. The person who the spouse of the one seeking the Sacrament of Baptism cannot be the Godparent.
This rule is similar to the one that says that the parents of the one seeking the Sacrament of Baptism cannot be the Godparents.
It appears that this may be one of those unwritten rules.