Dear @Red_Tapir
Welcome to the R/O forums!
We're glad that you decided to join us here, and we hope you find the support on the forums useful.
I'm sorry to hear about your experience of abuse from your sister. I can hear that you're feeling quite invalidated by your family around the abuse, which would be a tough thing to experience.
I know you mentioned that the abuse was in the past, but I just wanted to check, do you currently feel safe at home? Our service has a duty of care to report any instances of child abuse, which you can read more about here (scroll down to see section 3 on mandatory reporting).
Also, I'm not sure if you've heard of 1800 Respect. They offer 24/7 confidential information, counselling and support for people impacted by abuse. You can contact them on 1800 737 732 or visit https://www.1800respect.org.au/