Sansa was a name created by
George R. R.
Martin for the character so unless he makes a statement about how he created it any further ideas of its origin are speculative.
However, one speculation would be that it's an alteration of Sancia. That might make sense as in Renaissance Italy
Lucrezia Borgia’s brother Gioffre (1481-1517) married Sancia, daughter of the
King of Naples. Perhaps
Martin unconsciously remembered that when he was creating the characters Sansa and
Joffrey.
For more information on the use of "Game of Thrones" names for real babies, here is a link to my column on the subject:
https://omaha.com/entertainment/evans-game-of-thrones-may-be-over-but-its-legacy-lives-on-in-names/article_b92d1109-2206-51ec-919a-1fd73ac1d4b9.html