Little steps in the right direction, Grace, good on ya!
I am astonished by how family seem to take leave of their senses when it comes to presenting us to small nephews or nieces, as if we're going to leave them traumatised and scarred for life!!!

My sister and her husband were highly reluctant to tell tell my little nephews (6 and 8 years old). The reasons? (1) the trauma, (2) that it might give them ideas, (3) that it could activate some transgender genetics buried in them. I mean really, what rubbish!
Then they were told, a few weeks ago, just prior to my visiting them in London. The older one was shocked and cried, and the little one laughed. I think these are valid reactions. When I turned up they were a little shy for a few minutes, and then realised that Uncle A was now simply Aunt A. By the end of the weekend they were casually referring to me in the feminine.
I fully recognise and respect that parents wish to protect their children, and I treat their concerns sensitively. But kids have astonishing mental plasticity and are much more accepting of change. As for your sister's therapist, I am astonished - words fail me.
You're doing all the right things, in my opinion.