Hand being forced

So I feel no love towards my moms side of the family but that's besides the point. My cousins twins are being baptized on Sunday and I'm being forced to go. I have tried sparing my mother's religious feelings till I move out but its getting hard. I made excuses why I cant go to the baptism but to no avail so when I tried explaining that I'm an atheist I was told I was being ignorant and I'm just going through a phase, their family I have to support them and if I really feel that way then I'm not allowed to celebrate Christmas with my immediate family. Like many others I love christmas for the aspects that arent religious.