Here is an excerpt from IELTS OG.:
For example, when my cousins were backpacking around the world, they were able to reassure their family and friends that....
I find the use of family in its singular form very strange, because obviously the author was talking about more than one family, since cousins have at least two families.
I suspect this has something to do with family being a collective noun, but I don’t think it is a good justification. It isn’t wrong to say one family, two families, is it?