Dear RR18, so sorry for what you've been through. Both you are your mom need a therapy and you both need to take some sessions together.

Your mom, never coped up with the trauma of being attacked by sister. Instead, she just swallowed it and never dealt with it. You deserved unconditional love and support from your mom, but since she never dealt with her trauma - you were made to feel guilty for something you had no fault. That has given you a very unhealthy behaviour of dealing with things by apologising.

Dear, pls stop apologising for no fault of yours and start taking action. Pls find a therapist which both you and your mother are comfortable with.

Good luck!!