People break in different ways, under different amounts of pressure. Same event(s) happens to 10 people?
1. PTSD
2. PTSD with Borderline features
3. Abuser
4. Psychopath
5. Absolutely Nothing Wrong
6. OCD w/ Narcissistic features
7. Substance Abuse / Addiction
8. Generalized Anxiety Disorder
9. Delusional Disorder
10. Eating Disorder
Same events happen to 20 people? Can end up with 20 different sets of problems. Or none at all. 50 people? 50 problems. Or none at all. If you flip through the DSM? At least 1/3 of the disorders in there can be caused by trauma, if not more. PTSD requires trauma, but it's far from the only thing that trauma can cause.
So your siblings, if they experienced the exact same things as you? May have PTSD. They may have PTSD-primed, that's just waiting for a stressor to kick it into gear. They may have 1 of over a dozen trauma & stressor disorders. They may have 1 of over 50 other disorders or conditions that can be caused by trauma. Or they may have serious struggles and challenges that don't meet diagnostic criteria. Or they may have nothing wrong with them at all.
Why 1 person goes 1 way, and 1 person goes another way? We have no idea. Yet.