I'm not sure how you get PTSD from dealing with other people's crap - simply put if we want to be in relationship with other people we need to, from time to time, deal with the impact of their stuff on us. We don't need to adopt it as our own, carry it for them or take it on board but we need to acknowledge its there.
Good therapy will help you sort out what's your stuff and what's theirs. For example, someone did something to me that was very wrong (not talking about PTSD inducing abuse wrong, but a clear violation of my boundaries). It triggered something in me that caused a huge reaction in me. The trigger, and my reaction was all my stuff, yes triggered by his behaviour, but mine. The original thing he did wrong was his stuff - to acknowledge and apologise for, which he didn't and we have a very distant, superficial relationship now (me tightening boundaries in response to someone who didn't respect me).
So, your parents not believing you or accepting you have PTSD is their stuff, you can't control what they do with that. You can own the impact is has on you and how you respond to them and the kind of relationship you have with them (your stuff). Being impacted doesn't mean carrying their their crap, it means deciding how you want to respond.