This is a question that has been burning a hole in my mind for years.
As a teenager I was in an abusive relationship. When it all came out into the open his family told everyone I was a liar and crazy etc... It that instance I understood that my abusers family didn't want to accept the glaringly obvious truth, but I have seen other instances of abusers having people in their lives who totally excuse or overlook their abusive tendencies that really baffles me.
For example, I know someone who is abusive toward their siblings. This person clearly has some emotional issues that, for whatever reason, means they treat their siblings incredibly badly and will go to rather extreme lengths to emotionally and psychologically abuse these siblings. Things like using manipulation to pit them against each other and telling extended family members lies about them in order to isolate them by making them seem like bad people, and even attempting to isolate the siblings from their parents. This abuser has a fiance and this totally baffles me! The abuser is, as many abusers are, very good at being charming and hiding all of their behaviour from other friends and family members. But the fiance has been around for a long time and is surely aware of the way the abuser behaves. The abuser is always in conflict with the siblings after their behaviour is found out which highlights the issue of what they have been doing. Its not as if there are no clues. Obviously the abuser will go to hell and back to hide their behaviour and will try to turn it on the victims (in this case the siblings) but the issue is so obvious that I don't understand if the fiance is just unable to see it or totally in denial. Especially when huge rows erupt within the group of siblings because of the abusers actions. The abuser is the common denominator so it just becomes so obvious that I don't see how it could be overlooked. The abuser is a little controlling over the fiance and rather critical, but is not abusive toward the fiance in the same way as the siblings. It seems that the abuser targets the siblings specifically and doesn't behave this way toward anyone else.
I know another abuser like this who targets a sibling and who has a fiance who totally overlooks all of the abusive behaviour. This person seems to have anger issues that affects everyone in the family to some degree but seems to save most of their abusive tendencies for one sibling in particular.
I know these families very well as you can tell. These are just two examples as well as my own experience of having been in an abusive relationship. I'm just totally baffled by how some people can seem totally in denial, overlook the behaviour or just don't see it. The fact that these two individuals that I know have fiances that seem oblivious to the chaos that their partners create within their own families totally dumbfounds me. I think it is because unlike family members, fiances can choose whether to be with this person or not. Well even family members can choose, but it is of course a different kind of choice iyswim.