Godmother

Assalamou alaikoum dear sholars. Jazakoum Allah kheiran for this great website. I am a muslim convert and I would like to know if it is possible for me to become the "godmother" of my brother's daughter. My family is Christian Catholic and Catholics have this tradition of designating a man and a woman as godfather and godmother to a newborn. Traditionally, their responsibility was to raise the kid if the parents die and to swear to raise them as Catholics. Nowadays, priests do not ask anymore to swear to the catholic church, but being godparents is more of a role model and a sign of love and consideration from the parents.I would like to know if I can accept to be the godmother of my niece or if it is prohibited in Islam to take part in a Christian ritual.