I wonder if your T was not doing that to stiffle your emotions but more to help you recognize that what your sister was doing to you is not healthy for you. That you need to consider yourself and not allow yourself to thrown by what your sister is doing.

Perhaps because that is not your responsibility but your sisters. In other words you can't make your sister change, she needs to change on her own and you should not carry that guilt. Maybe your T wanted you to be aware of that. Just some thoughts.