40 years old and just found out I'm adopted

I just found this site tonight. I learned 2 days ago that I was adopted. I was raised to call my birth mother my sister. She was ostracised by my family and now lives in Maine. She has had other kids and recently one of her daughters almost died from a drug overdose, so she ended up starting the process to adopt her daughter's daughter. I was telling my other sister (there are 3 girls and 2 boys, all older than me) about this adoption and this enraged her. In her rage, she told me I was adopted and showed me the documentation to prove it in an effort to make me mad at her for adopting her grand daughter. Her actions have made me distrust the entire family. The woman that raised me (not sure if I should say mother or grandmother), died in August after a long fight with lung cancer, during which, I was the only one of my siblings willing to help her. Before that, one of my other sisters was diagnosed with MS and I was the only one willing to help her as well. I gave up the last 20 years or so taking care of a family that had no problem with lying to me. Now they're all trying to tell me that they all wanted to tell me, but we're talking about people that have never had an issue with breaking rules. On top of that, I would still not know if I hadn't presented the information about my birth mother to my sister. Now I feel somewhat guilty for the way I've reacted to the news. I've completely shut down to everyone in my family, including the sister I've been taking care of all these years (much of that shut down has to do with the fact that she told one of the people I grew up with about my adoption and also told him that I was the product of incest between my birth mother and the man I was raised to call my father. I have learned that this was a lie). I'm not sure what to do now. I look back on my life and it seems obvious in retrospect, but you never expect your family, especially your parents, to lie to you like this. I think I might benefit from some sort of therapy or an adoptee group if they exist. Any suggestions on where to go from here?