They interviewed the family extensively. Wouldn't they be the ones who would know?
Yes, family members would know of any respect / disrespect confrontations- providing that the victims informed them of the dispute.
Some people might not inform family members of such confrontations due to personal embarrassment about the nature of the interaction (romantic, drug usage, financial, implication that teenager is not obedient, implications that they "cant handle" a seemingly minor dispute etc) or simply not wanting to bother their relatives with the matter.
Likewise, even if the family members know of a dispute, they might not know specifically with
whom the conflict had occurred. For example, one of my family members had a minor conflict in the past with a local well, scam artist posing as a contractor.
Relatives knew of the conflict and know that she had eventually paid him a relatively small sum of money just to “go away”. Yet, many, if not all of the relatives, have no idea who “he” specifically was. Rather, they know him only as the "contractor guy".
If, God forbid, something bad happened to my relative, knowing of the dispute- then examining local bad news types known to be on again / off again home improvement contractors for a connection could be beneficial. Likewise, if my relative had not told relatives, maybe a local bad news type was known to trawl around the neighborhood approaching women with contract offers..... .