I can't imagine how anyone could criticize you or the pups over that, no one is to blame. As many others have posted, most squabbles happen over food or possessions, and as Kiara gets older Nala may feel her place in the pack is threatened. It's good your parents decided to give Nala some special treatment, that would be very reassuring for her. Others have given you great advice as well! Just don't be too discouraged, I'm sure you are already taking all the right steps to help the situation as best you can. I imagine Nala not feeling well contributed as well, maybe she felt even more threatened than usual since she wasn't feeling well.