It is definitely not against the rules to belong to an NPC and a service org. One thing to consider however and the only thing that may keep your other sisters from being supportive is how time consuming is your service org ? Honestly NPC sororities generally take up a good amount of time and if you aren't going to have enough time to dedicate to each I say choose one over the other, if your service org just meets once a week and has events everyonce in a while.. then by all means go for it and your sisters should have no problem with you joining an NPC.
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