I find this confusing; do you feel disorientated by what happened?

I am confused about why your therapist is negotiating with your sister about sending you a birthday card, isn't that a discussion which should have taken place between you and your therapist directly? It is an act which potentially requires some delicate discussion around boundaries, both yours and your therapist's. The confidentiality breach is the least significant factor for me - I would be far more concerned about the lack of control I had over my therapist sending me something. I don't understand why your therapist thought it was appropriate to enter into this triangle. Regardless of whether you gave the number or not, it was your therapist's responsibility to step outside of the dynamic.

I am confused about your sister's interest in inserting herself into your therapeutic relationship. Similar to what una has said (and I know you haven't liked what una has said), it feels like your sister is playing puppet with your therapist.

And I am confused about what motivated you to give your therapist's number to your sister. Did you know why your sister wanted it? As much as this would make me cringe and I would find it really uncomfortable, I would spend time uncovering what was happening for me that prompted me to give the number. Was I testing my therapist? Would it be thrilling to receive the card and that was my primary interest? Is the idea of the card thrilling but the reality of the breach unsettling and so I have tripped myself up? Have I sabotaged trust because I can't maintain closeness in a relationship? What am I hiding when I facilitate a drama like this? Uncomfortable questions for sure, but for me, they would be fruitful in my development rather than focusing on my therapist's behaviour and what she did wrong (and I say this as someone who likes to blame my therapist for everything).