I have PTSD and agree with you, but thats why I vote YTA. Only a doctor can diagnose someone with PTSD and its not OPs place to police their sister's mental health. They need to encourage them to go see a medical professional. Doctors don't hand out PTSD diagnoses like participation trophies, so if she's making stuff up they'll be the ones to figure that out and get her the help she actually needs. Until then, it could do serious harm to belittle the sisters experiences because what if she did have a traumatic event and she just didn't confide in OP? For example, its well known that sexual assault victims don't often come forward because of shame.