Siblings of adopted child who contacts birth parent

I am hoping that someone on here can guide me to the correct location to find support for the siblings of a child put up for adoption. Last year through ancestry.com someone contacted my family and said my mother was her birth mom. This was a huge secret my siblings and I knew nothing about this child who was born before my mom married my father. The issue is that we siblings have nowhere to turn for support. We are told to consider how the adopted child must have felt all these years not knowing their birth mom. Guess what? I also did not know my birth mom had this secret. My world has been shaken. This child knew from a young age of being adopted but this is news to us. Is there a support forum for siblings of adopted children to share their feelings and experiences?