The sanskrit word 'sakshi' means witness. And, the definition of witness is 'a person who sees an event'.
True to her name Sakshi - sees the events taking place around her. For example, Sakshi saw Devanshi emerge from the lotus pond post-leap. Then she saw Devanshi fall into the hundi that was in the tempo destined towards GWM's temple. She overhears Sarla's secret that one of her daughter's is not her own.. She sees Devanshi being kidnapped. And now she knows that Devanshi is the adopted daughter of Sarla. I have not watched all the episodes - so I may be missing other parts of the story where Sakshi acts as a witness.
Sakshi is not only Devanshi's sister, best friend, and room mate - she is also our eye and ear regarding events where the people involved think that no one is watching or listening! 😊