According to William Stevens' FindAGrave memorial, his parents were Eli Thompson Stevens and Susan Elizabeth Richards Stevens. Cordelia's parents were indeed Caleb Perkins and Cynthia Ann Whitaker Perkins.
Caleb and Cynthia, just like you already know, are the parents to both Cordelia and Sara. Sara was born on January 20, 1844 and died in 1904.
I found a record that gives me marriage info for Francis marrying a "Bell Perkins", whose birth year is the same as Sara's. The marriage date is October 30, 1865.
So I would say yes, Francis Marion Stevens married Sara Belzora Perkins, sister to Cordelia Whitaker Perkins who married Francis' brother William Wallace Stevens.
Also there could be an error in Francis' name. The record I found has his name as Marion Frances while you have it as Francis Marion. I am trying to find a correct spelling for you but the woman who provided the information on FindAGrave passed away last February.