I had the same dilemma. We asked my sister and her partner to be GP to DS3 at a family aprtner, only to overhear her partner joking about how great it will be when he gets up and renounces the devil-since he sees himself as a pagan, that would really test the faith of the Christians.
I was furious, but my sister is so precious that I knew it would cause a family rift if I called them on it and made it an issue.
So for 4 years, he remained unchristened, until last year, the others were looking at their Christening presents and talking about their Godparents, and he cried because he wanted Godparents, too.
I just arranged his 'baptism' for the next available date, asked a good friend of mine and an elderly lady who came to help from the church when I had 3 under 4 and has been coming ever since, and I told my sister that I wanted to make sure they had GP who would be around every day (she lives 5 hours away too), and who would teach him to pray and follow his bible.
I layed it on a bit thick, but I gave criteria to being a GP that she couldn't match, so she had no way to complain.
In the end, she couldn't be bothered to make it to the church that day (despite it being near DS's birthday, and summer time, and she came up to see my mum the next weekend), and she didn't even bring DS a card.
His GPs have been wonderful, so I'm not sorry for my decision.