IN this case what happened is , there should have only one LD quota seat available while booking and that is allotted to your wife and the second passenger is given RAC in GN booking. Its absolutely fine as there is no age criteria for LD quota. BTW when is your date of journey?
As Kamesh Sir said find if any LB seats are available and book it for your mother,but it is not guaranteed that you will get in the same coach.