There is no logic.bcz preeshti are still young while their sister pragya became old.but since everyone considers preeshti as prachi's aunts they can also be considered old when they attended lakshmi's wedding.
Why do you say that prachi was not born then?at the time of lakshmi's wedding pranbir were already married.
It could only be logical if Preeta-Srishti were in their 50s when they attended Lakshmi's wedding otherwise its no acceptable for Lakshmi to be with their younger self in their wedding and few months later adult version of their niece who was not even born then.