Ok, I'm probably not the most "in the know" responder so far

, but here's my answer (others please jump in if I misspeak): The final result comes down to how each group does their voting and makes their bid list. And we don't know how every group does that, so I can only say that yes, basically, what you said is correct. Although I don't think "points" have anything to do with anything at actual bid-matching, I think it's just that quota (how many each chapter can take) has been set, and the computer will match everyone on XYZ's top bid list with everyone who ranked them first, if they go through their first list and didn't get quota, the computer starts matching people from the "rest" of their bid list with rushees who put them #1 until they reach quota. After they reach quota, anyone "left" on their bid list that had ranked them #1 will likely be getting a bid from their #2 or #3 (if they suicided XYZ, they won't get a bid).
Like someone else said, we won't have any knowledge whether XYZ is "less likely" to rank her high if they know she's a legacy somewhere else. I had plenty of sisters who were legacies other places so I don't see why that would happen, but again, you would be more likely to know that from local sources.
So she will need to decide whether if things don't work at XYZ, she would want to take a bid from ABC. If she Prefs there, she is likely to be high on ABCs list as a legacy, pretty much assuring her an ABC bid if she wants it (again, depending on the org, I can't guarantee this is true across the board, but that's my impression).
ETA: Wow, I took too long to respond, the thread's already done, it looks like.