Dear Marina, I am glad you care a lot for English plural forms of nouns. You are probably learning English as a Second Language.
The noun police is followed by a plural verb because it is a very big group of people. For one Individual, we talk of a Policeman or a policewoman. For the word Family, I need to check with other coleagues.
Chris, a Student
Kigali Institute of Education
Faculty of Arts and Languages
English and Literature With Education (Year III)