I think the 'only' problem you have is to decide what you mean by a 'family'. Once you decide that then displaying the related information will be easy. I think your problem is that for one scenario you decide that it is best for a Family to be Mom, Pop, and the 3 kids. Works well. But then in another scenario you decide that the grandparents should also be identified as 'that family', which works great. But then messes up your earlier scenario. Then you want cousins to be part of the family to satisfy another need, but that makes the finding of the simple 'core' family very difficult.
It think the Filemaker part is relatively (ha!) easy - once you decide the family unit.
I suspect that rather like a family tree you could have one join table per generation. So you have every a record in the join table has the ParentID and ChildID. Obviously one parent can have many children, and children can have many parents. But then, depending on your definition of 'family' you could choose to add a join record for the Husband/Wife IDs, so looking at either Husband, Wife, or kids records will show all their family members. If you want cousins and step-siblings you can add join table records for as far out as you want the 'Family circle to go.
I believe it will technically work fine. Whether is is humanly manageable of not, though...
I set up a contacts management system for a children's ministry. What I quickly realized was that a given child could easily be a member of more than one family due to divorce, remarriage, etc. Considering relatives not part of the immediate family would be another, similar case. Here's what I set up for this:
Family::__pkFamilyID = Family_People::_fkFamilyID
People::__pkPeopleID = Family_People::_fkPeopleID
People::__pkPeopleID = PhoneEmails::_fkPhoneEmails
Fields starting with __pk are primary key fields defined to auto-enter serial numbers.
Fields starting with _fk are foreign key fields defined as number fields.
Mailing address info was recorded in Families.
Name, age, gender etc. was recorded in People.
And the PhoneEmails table allowed me to list as many phone numbers and email/social contact site addresses as I needed for each individual.
A similar table could be added in your case to list a flexible number of "interests" for that individual.