Why Does "Family Tendency" Mean?

I think that family tendency means that it is something you tend to do as a family

What  you  tend  to do  as  a family  together

It means there is a tendency in the family for something to happen,( ie a disease,)  Many tendencies are genetic but ,then because we are a product of our genes and also  of our environment then the way/s that someone is raised also can produce a family tendency for things to happen.

Something inherited or copied from a particular family by someone who is a member of it.
  Seesee