Nope, if you see the cabin available you can book that cabin anywhere. The inventory you see on the various online agence sites is actually the cruise lines inventory. When agencies buy blocks they don't buy specific cabins. Everyone sells from the same pool of cabin inventory. You can book there or through the agency your family used or even through the cruise line direct. As long as that cabin is available you can book it anywhere you'd like.
Just be sure when you do to give the agency you book through your family members Bk'ing # also and request the cabins be cross referenced for dining. That way you'll all be assigned to the same table for dinner seatings.