I expect that, among the many things that family member(s) may be asked about Bryan's life, surely someone, my guess is the sister who suspected him of being the killer, will be asked and will be able to corroborate evidence that BK had purchased a KaBar knife from Amazon, and that it was never seen in the home again, after he left for Washington, and yes, the sheath that they will surely be shown in court does look like the sheath to the knife that he bought. I also expect they may also be asked about his peculiar behavior upon returning home after the murders, about if it was normal behavior for him to wear gloves in the house, and normal for him to bag his trash and separate it from the other family members', and I expect the sister to be asked why she suspected BK enough to search his car.
I do not know how helpul BK's parents may be if called to testify, and I would expect the state to tread lightly with them, so as not to come off as unsympathetic to them, but I expect the sister's testimony to be very helpful to the state's case. JMO