Boundary Script For Chatty Sister

Yesterday, I discussed with my sister that my teenage daughter had just confided in me that she is having a hard time. I called because I am scared by what I learned about my daughter and my sister has raised two girls. She had some helpful suggestions and I was glad that I talked to her, until today…when my sister called and said that she told her daughter what’s going on and that her daughter wants to call mine to offer support. My own daughter doesn’t even know that I talked to her aunt or to anyone and her cousin, while I love her dearly, is still pretty deep in her own teenage issues and I am not sure if she would be a helpful resource or make the problems seem larger. I didn’t tell my sister not to tell anyone, but now I guess I need to, before she tells grandma and it awkwardly comes up in front of my daughter. I just assumed she would keep it confidential, as she has always been private about her daughters’ issues. I am annoyed that she told and this makes me not want to confide in her. Every year or so i am caught off guard with her sharing things I didn’t want shared. For a long time, I just kept her out of the deeper details of my life, but am trying to form a closer relationship with her. How do I script a boundary that honors my need for confidentiality without making her feel like she can’t ever tell her family about anything going on in mine?