You might have to make some alternate plans so you can attend preference that one time -- hopefully it will be held in the evening.  Once you're in a sorority though it shouldn't ever be a problem.  In my experience religious obligations are 
always respected.  
I'm certain that all NPC sororities have non-discrimination policies regarding religious beliefs.  No one is going to think less of you and it's not going to lessen your chances of getting into a sorority.  Not to worry.