Is it normal my parents are angry at me for taking my sister in?

As soon as she turned 18, my sister moved in with me, just to get away from our psychotic parents. It's something we had been planning for a while without telling them. She had been sending her stuff to me over time so that she wouldn't have much to take with her, only leaving stuff our parents wouldn't notice was missing, and didn't announce it to our parents until she was actually leaving. She waited until they went somewhere then hand some friends come over to quickly box up the rest of her stuff before they came back. She put some of her stuff in her car and had the rest taken to my house by her friends. With all of her possessions taken care of, she waited for our parents to come home, told them she was moving in with me, and left. This may sound cold, but if you knew our parents you would agree they don't deserve any better. Anyway, this caused them to flip out. My mom called me, told me I was "stealing our baby", and my dad took the phone and demanded I send her back. I told him that wasn't going to happen, my sister lives with me now, and that's all there is to it. Then they tried telling me that she couldn't have really moved in with me, that she was only visiting. I don't know what made them think that would do anything. I told them she must have intended to move in with me because she had already submitted her voter registration with my address on it, and that if they had looked at it they would have already known this was happening. They got furious and started screaming things at me I didn't really listen to, while I calmly tried to explain that my sister is no longer under their custody and lives with me now, and there was nothing they could do. I couldn't believe this wasn't getting through to them. My dad told me I was lucky he didn't come over to my house to strangle me and hung up. I was hoping that would be the end of it, but it wasn't. They tried taking legal action against me. Of course, that went nowhere fast as it was repeatedly explained to them that she's an adult and can choose to live with me if she wants to. Finally, my mom called us just to explain she wishes she had taken our dad's advice and aborted both of us, and hung up. If they call again, I probably won't answer it. I don't need them or their harsh words. What could trigger so much hate like this?