Do they have their own BAEC accounts? I presume so if you have added them to family & friends. I see the ability to add / manage my third party nominees as follows:

1. Log into the BAEC account for the main booker of the trip (elderly relative)
2. Go to Manage My Account
3. Fourth menu item up from bottom I see "Manage Third Party Nominee"

Then you can add, edit and remove up to 5 third party nominees. Add you as the nominee and then you should be good to go to help manage their trip.

NB When I went to make a return 241 leg booking at 1am some time recently my third party nominee password on my wife's account had been changed randomly but I was able to reset it while on the phone and it was instant, so adding the third party nominee should be so as well.

Good luck