You're partially correct: Keiko's father, Hiro, was unable to attend the wedding as it was in deep space and as Data introduced Keiko & Miles he was asked to share this special day with them.
Data, not being what you might call best friends with either, still had a huge part in both of their lives by bringing them together. As standing at their side was out, for the aforementioned not BFFs, as was officiating which is the Captain's duty. All that was left would be walking the bride to the altar or "giving away the bride."
Giving away the bride is an antiquated tradition from the days when women were their father's property until they got married and became their husband's property. Today parents or families instead give their blessings in traditional and non-traditional ways where the bride may be walked down the aisle, sometimes by both parents or by a close relative.
Since that is the case today it is not hard to understand Keiko's request to Data would represents her life on the Enterprise prior to starting a family with Chief O'Brien. Data, her analytical confidant of whom she closely associates with her next step in life, would be the best of the choices at hand if she feels she would like to carry on that tradition.