My sister has been having nightmares every night for the past week. Help me please.

So, my sister has been having nightmares every night for the past week. Quick answers, yes she does have trauma from a couple years ago, we both do, I sort of suffer the brunt of PTSD though, her emotions aren't as connected as mine are. It is very hot out, every night, it's a Midwest summer so you already know it's temperatures of 80 or higher every night. She also has just recently started marching band. We both do marching band it's just a bunch of little to no stress practices on Tuesdays and Thursdays, mine are inside, hers are outside. Really, I'm concerned because it hasn't been happening to me and we're twin sisters of the same age. I'm concerned and I don't know what to do because what if she has like a condition that's gonna kill her or something. I've told her to tell our parents but she won't so I'm gonna tell a bunch of strangers online without using her name, because privacy. She's never really cared and I feel awful going behind her back but I just don't know what to do, and I'm concerned and I need help. Can anyone please tell me what might be going on?