Hi Andy,
If your sister was initiated into the local and that local became a chapter of TPA, she is automatically eligible to be initiated into TPA. In fact, she should have been conacted about it when the TPA chapter was installed. In any case, I would suggest that she contacts National Office and explains the situation to them (info@thetaphialpha.org or 1-877-THETA-PHI). Just out of curiosity, what school did she go to?
Good luck and please let me know if you (or she) need any more help with this!
Edited because I put the wrong phone number 