Me again (sorry...I keep thinking of stuff to add).
A lot of the NPC sororities do have a small blurb on "alumnae initiates".
For example...
Alpha Phi:
Alpha Phi Fraternity offers alumnae initiate membership to outstanding women who are committed to the principles of friendship, sisterhood and support. Whether they chose not to take advantage of sorority membership as a collegian, did not have the opportunity at their college or did not attend college, they can become Alpha Phis. The proposed candidate cannot be an initiated member of another NPC group or currently meet qualifications for collegiate chapter membership.
Gamma Phi Beta:
Gamma Phi Beta welcomes into membership qualified women who never joined a National Panhellenic Conference (NPC) sorority in college. This program allows alumnae members to invite outstanding women to share the pride of being a Gamma Phi Beta. After a short training period, the candidate is initiated at a nearby collegiate chapter.
If you are interested in recommending a woman for membership or in becoming a sponsor, contact the member relations team at International Headquarters.
Alpha Sigma Tau:
Alpha Sigma Tau offers an alumna affiliate membership to women who did not have the opportunity to join a sorority when they were in college. Close friends and relatives of sisters who practice the ideals of Alpha Sigma Tau may find out more about participating in this program through the alumnae association in their area, or by contacting the Director of Alumnae.
This should get her started off nicely...but I think
alumnae initiates is the way to go for your Sister In Law's situation.
Let us know how it all turns out!!