Very often, symptoms of personality disorders are evident by teens and early adulthood. They *can* manifest later but it isn't as common. Still, some people go literally decades before being diagnosed if they are at all. There's a large spectrum in personality disorders - some people are relatively functional for a long time and others have their lives imploding so much so they're forced to get help.
I really can't fault Bill and Sandy for what I'm guilty of myself. Ignorance, love, hope. When you don't know that someone you love is personality disordered you're literally racking your brain trying to explain their actions and behaviors. And when that loved one is subjecting you to near constant psychological abuse, it's very difficult to maintain perspective. You find yourself enabling them and rationalizing their behavior, caught on a roller coaster of ups and downs, without knowing how you got there or how to get off the ride.
I'm there with my teen daughter right now. Because her dad is personality disordered, and there is some research to suggest there *may* be a genetic link, I'm hyper-sensitive to any traits she displays that may be consistent with PD. Unfortunately, that's exactly where we're at now - she's displaying a lot of borderline symptoms. So, back to counseling we go. But we're only there because I know what to look for and her dad was dx'd. If there wasn't that history, I'm not sure how much I would chalk up to 'normal' teenage drama since, even knowing what I know, I hope and pray I'm wrong on my suspicions and it is indeed just normal teenage drama. I'm not Bill or Sandy but I do understand how scary it is to have a teen in a full on rage and threatening their sibling; I do understand the hurt and confusion that no matter how much you love them and no matter what you do for them, it never seems to be enough for them to feel loved; I do understand the pain and anger of being manipulated, lied to, and verbally abused by your own child. And unfortunately, I do absolutely understand the revolving ride of being hopeful when she's happy or behaving 'normally' and hopeless when she's not.
It's a really yucky place for a parent to be.
JMO and FWIW