Hey, Corina.
This is my first post, but I have been following from the get go. Impressive...
Anyway, I agree with Theta Sig AGD and FuzzieAlum. I say go ahead with the meeting without her. Sounds to me like she's pretty much made up her mind about wanting to be in an LGLO. Let her go if that's what she wants. She can't be an asset to NBO if she doesn't really want to be there. Also, I have never heard of any local social sorority affiliating with an LGLO or any other GLO outside of the NPC. I'm not saying that it's impossible, I've just never heard of that being done. Since the majority of you decided to affiliate with an NPC sorority, I think that's what you should do. She can't expect NBO to go back on that at this stage in the game.
A word of caution, your sister may not even be eligible for membership into the other organization because she is already an initiated member of NBO. She needs to make sure that this new organization does not have any stipulations against being a member of another social GLO ~ whether she deactivates or not ~ before she makes any hasty decisions. In the end, she may run the risk of not being in any org.
Just something for her to look into....