Check out "The Hardest Part." It’s a great album and there is a "hidden track" about 30 seconds after the last listed track, "Feeling That Feeling Again," that gives Allison’s version of the events of that night.
I don’t think that Shelby and Allison witnessed the murder/suicide but they were at home at the time.
Allison has just had a tragic life. She married Steve Earle a few years ago and had an autistic son with him. That experience influenced songs like, "Mama Let the Wolf In" on her most recent album, "Down To Believing," which is also a very good album, less country, more rock.
Since Steve is not the kind of guy to stick around and raise an autistic son, the marriage ended in divorce. Allison is a tremendously strong person to have endured all this and be able to carry on. She’s a great singer too. I’d recommend the two albums I’ve mentioned and then explore some of her others if you like what you hear.