Sister-adoptions are nothing new to my chapter!!!
One of my sisters had a really horrible sister-mother. She was MIA for most of her daughter's pledging semester...and she wasn't even there for Initiation Day! I could tell that her daughter was hurt. Her Initiation pillow was there on the table...but no sister mother. The sister mother ended up turning in her Badge and she refuses to say hi to any of us when she sees us around campus.
Because I'm in charge of compiling the family trees (dating back to 1992!), the daughter asked me to acknowledge her adoptive sister mother as her real sister mother because she doesn't want history to reflect the fact that she was literally abandoned.
Honestly, prospective sister-mothers like that don't deserve sister-daughters! But anyway...getting back on topic...
So I guess it depends on the chapter. And it depends on your situation with your first big. In my chapter family tree, I have a bunch of "adopted bys". Some girls are still great friends with their first sister-mothers, even if they may not be in good standing with the Fraternity. I made it clear that it was totally up to them. You are a member of your organization for life, so even if she's not physically active in the organization at this moment, at one point she was. She'll still be your sister whether or not she's in good standing.
But I think it's great that you have a potential adopted big! I totally understand how that bond is important...
Good luck and let us know how things go.