"Those girls are my daughters."
Translation:Quelle ragazze sono le mie figlie.
For plural, you can put the article. But in some cases it's not needed. For this sentence:
Quelle ragazze sono le mie figlie = Those girls are ALL my daughters OR Those girls are my daughters, not yours or theirs.
Quelle ragazze sono mie figlie = more generic.
Please note that if the relatives are the subject and not the object, the article for plural is needed.
"Le mie figlie studiano all'università" (Mie figlie studiano all'università would be wrong)
This site has some clear rules and examples .. it can be quite helpful (although it does not discuss the plural as object case)