I feel completely betrayed

I found out yesterday that I am pregnant! It wasn't directly in the plan, but I wasn't exactly preventing it.

I told my two very best friends once I arrived at work and they promised not to tell anyone.

Of course, I told my boyfriend.

I wanted someone in my family to know, but I didn't want everyone else knowing because I am only 6 weeks along. I don't want it being common knowledge until at least 12 weeks. I told my sister, Cara, because she already has a child and I wanted someone close to share this with.

I have another sister, Jillian, who is bipolar and has other mental disorders. She has been staying with my parents house because she is unable to hold a job or a place to live. I love her dearly, but her mental disabilities and refusal to get medicine makes it impossible to be close to her. I was at my parents house visiting when Jillian started a fight. It was over her stealing things from me that I accidentally left at my parents house. I was very calm and trying not to provoke her. She physically attacked Cara while she was 7 months pregnant, and I didn't want to take the risk.

Jillian said "You are lucky I am not a vindictive bitch because I have some dirt on you." Then glanced at my parents, daring them to ask me what she knew. She kept saying things like "Why are you being so calm? Why aren't you yelling at me? What has recently changed to make you act this way? What are you hiding?" I knew that she knew and she was making sure my parents would ask questions. I'm independent, so I couldn't be in trouble and I am in a serious relationship. I just didn't want them to know yet.

I went to Cara and asked her if she had told Jillian. She said that it was an accident and that it had slipped. I started crying. I wanted everyone to find out on my terms and on my time. She said that I was being absurd and needed to get a handle on my emotions.

I feel betrayed and like I can't trust anyone in my family. Do I have a right to be angry with my sistera? Or am I over reacting? I don't know what to do.