In the fic, the main exposition about the Darcy-Fitzwilliam family history (thus far) comes from Wickham, always a dubious source, but he's mostly not lying outright. Catherine's grandfathers, Lord Harcourt and Sir James Darcy, were great friends who saw themselves as also great trailblazers because they were very into "the East Indies" in their way, and personally traveled to India (more specifically, to modern Kerala). The circumstances that led to them shocking their social sphere by bringing back a pair of beautiful and wealthy Indian women as their wives were never much talked about in the family, so Catherine herself only knows so much. Certainly, all concerned would have considered it a great impertinence for anyone (esp those outside the family) to actually ask about their motives.
Catherine does have the impression that her grandfathers both considered themselves quite noble for going through with the marriages, but that Sir James had a more realistic idea of the difficulties Lady Harcourt and Lady Darcy might encounter in their society. It's only an impression because she never knew him; Sir James died young, leaving Lady Darcy as a rich widow with one child, a very young son. Catherine did know Lady Darcy and was on good terms with her until she died, when Catherine was about eleven.