Parents Threatening To Kick Me Out Over Speaking To A Private Investigator

As the title suggests, I 18M have a sister 25F who is currently going though a legal battle with my best friend of my whole life’s dad, claiming that she got molested 15 years ago by him. While I know it sounds awful I wouldn’t believe her, she has been confirmed to have lied about stuff similar to this in the past and has a very attention needy personality and due to all the evidence and texts I’ve seen, I made the hard choice to be on the side of my friends family, as he is more of a brother to me than she is a sister to me (we have never ever been close). Recently the private investigator that was hired by the other family had texted me and asked me to give a character witness and give overall thoughts on everything, I told him I would and also told my parents that I was going to do so. My parents told me if I did, that they would kick me and disown me and keep urging me to not talk to him. I believe it is the right thing to do to give my honest opinion, as my sisters claims have been extremely contradictory towards themselves as well as having a ton of inconsistencies and proven lies already. Is this legal? To kick me out for simply talking with the PI about the case?