My Mother in Law made the christening outfit for my boy's. I had my dress from my christening but my husband said it was too girly! It was made with white velour and white silk.
Check out this site,
www.a special day.com they have real nice dresses and they are not to expensive. Another site is christeningbabygown.com, they have a few that are a lot of money but some are less.
Also our church said that as long as one God Parent was catholic then it was fine. Child #1 had a both Godparents catholic, child #2 1 catholic one baptist godparent, #3 both were catholic, and #4 one catholic one was baptist. All are on the baptismal certificate as God Father/God Mother title. You might want to have your sister see if that is true at her church. It night just be different rules for different diocese that the church follows.