I've been in the same situation, more or less.
I found the social media (Facebook, Instagram) of my T's sons. Well I didn't "find" it, I went to look for it.
I started following their social media postings to the point that it got out of hand because of the feelings it was eliciting in me. I felt super guilty.
I finally told my therapist, expecting her anger.
And she was totally fine with it. Barely suprised.
So are you sure your T is gonna be angry at you? Why would she be?
After all, what you read online about her daughter is part of public domain, no?
I think curiosity is very normal. You shouldn't beat yourself up about this.