Agreed with Permacultural, it's definitely possible to take of the therapist hat. I'm constantly noticing behavior/statements in people that is probably causing them issues but I don't bring it up to them because most people are not receptive to unsolicited advice and also because I don't want to be a professional for my family and friends. My job is very stressful, I work hard on not getting burnt out or having "compassion fatigue." I don't want to work 24/7, my home time is for happiness, personal care, etc.

How I achieve it - I just don't do it! Lol. I have certainly "diagnosed" my family with stuff in my own head but they're all super invested in not changing so what good would it do to bring it up? Like all other people, therapists or not, your sister is able to pick and choose what she says and when/how to say it if she chooses to. Some people use their education or title as a way of feeling superior to others, not saying this is your sister but just putting it out there.

As others have said, I think you should just address it with her however you feel comfortable. If writing is easier, write her a note. If you prefer to be more direct, I'd wait until she starts dolling out advice and say, in a nice way not sarcastically, "I really would like to talk to you as my sister. I already have a therapist."