Here's a good rule:
Don't say anything about parents and eating disorders that you would not say about OCD, autism, asthma, or a broken arm. It's easy. Try it.
Does this rule change anything in how you approach families or change your assumptions? If it does, then why? Why do you NEED these other assumptions?
I've found that every single thing that people say that DOESN'T follow that rule leads to poor treatment, poor response by parents, social stigma, and bad outcomes.
That rule doesn't prevent you from saying anything about the patient in particular - and actual things that person has experienced - but it starts you in the right place.
I still, after years of looking, have not found anything to be true as a group about families of eating disorder patients except that they SUFFER and that all want to help their loved one. Yes, I said "all." I haven't yet met one family that didn't want their loved one to recover, and if their efforts to do so seem wrong-headed or too weak or counterproductive then remember all the conflicting messages they've heard and the absence of support for families and the way the illness itself tries to alienate the very people who would want to help. Families facing cancer and injuries also struggle to respond well, but rarely are they faced with a professional world and a public that STARTS with wrong assumptions about their ability and motivation.
All I'm asking, really, is that one rule.