when you say your mother hadn't been abused i wonder...
one doesn't need events of 'objectively sickening severity' in order for one to legitimately feel traumatised.
trauma can be defined as intensely negative emotion that is not adequately soothed / held / processed by (m)other.
i don't know that there is anybody in the world who hasn't had that kind of trauma it is just that when it occurs against a basic backdrop of support then people are more resilient.

i don't know what the difference is between you and your siblings. most likely a combination of things...
some people are more sturdy or resilient due to biological factors (that is to say that they are slow to emotionally respond, their responses aren't as intense, and they are fairly easily soothed back to emotional baseline).
some people dissociate from their traumatic experiences all their lives... it could be that your brothers and sisters are still doing that because that is how they cope. it could be that they had similar experiences to you but that they have blocked them from memory.
it could be that environmental events happened to you that didn't happen to them. maybe you got sick and your mother tried and tried and tried to soothe you but your need exceeded her ability to soothe you. and so... she may have dissociated from your need in order for her to cope whereas she didn't need to do that for your siblings because they weren't as distressed.

it is most likely to be some kind of combination...

i answered you because i didn't want to ignore you
(like what happened with your mother)
and because it seems to me that this thread is about
why
why
why
people ignore others distress.
ESPECIALLY the people who we think are precisely the ones who should soothe it.