D.R.
Not sure of your religion would have certain rules. However, from my experience, Godparents do not have to be married or even family members. My godparents are my mother's brother and my dad's sister (NOT married to each other). I have known good friends be Godparents be for other friend's kids.
In a relious aspect, the role of the Godparent is to ensure the religious life of the child if the parents are no longer capable to do that. A family member is good to fulfill that role since they usually have the same religion.