^^If the group is a member of PHC on her campus she could not join an NPC at her campus later.
That being said - your sister is being very narrow minded and bigoted. It sounds like you love this group, and that this will be the perfect fit for you and what you want to do on campus (i.e. being an RA). Also, you've already seen that the group has a strong alum chapter. That's more than some of the NPCs at your school can show you, I'm sure.
You've already shown you don't just want the shallow things like popularity out of being Greek - if that was the case you would have signed your bid card after formal. You want a chapter you truly connect with and it sounds like you've found it.
So go for it!! Sign your bid ASAP!!!
