At the same time, I think, I'm getting angry over a piece of office equipment?
It’s not about the office equipment!
It’s about unresolved family of origin issues that you haven’t found a healthy way to address yet.
Maybe you were detaching just fine but the getting sucked back in part still needs work.
I personally disagree with your therapist that you should talk to your sister about her sexual abuse. Unless your sister comes to you and wants to specifically talk about that very private issue, then that issue of hers is none of your business.
Work on things on your side of the street with your therapist- resentment, bitterness, judgment, inability to let go of past hurts.