This is a usual question from the family having 2 or three members with the same nakshtra.First you need to understand that there is little scope for us to avoid such recurrence except in cesarian section births.
Going a little above Astrology as per the indian vedic system of knowledge and as known by me from my masters , each soul chooses the parents , the surroundings , situations and ofcourse the time of birth to suit to their karma fullfillment. hence it is not be woried whether 2 or three people are born in same nakshtra.
However i need to give you an astrological answer to your question. The only drawback with the same naksthra for close relatives is the disadvantage (or at times advantageous also)in identical malefic planetary transits for them. this may be severe especially in sadesathi like situations. hence it is quite often observed to give problems for the entire family . barring these there appears no severe problem in such cases. even the compatibility rules framed by great sages allow marriage of persons with same nakshtra except with the 1:108 pada relationship.
there is no cause for worry and they have come to work out their karma together. let us wish them success in their endeavour. good luck!!