'Polar bears will be consigned to history, something that our grandchildren can only read about in books.'
Members of a family are related in two ways: by blood (blood relations) and by marriage.
We are all familiar with words describing blood relations, such as father, mother, brother, sister, grandparent (grandfather/grandmother), grandchild (grandson/granddaughter), niece, nephew, cousin etc.
(A note on grandfather and grandmother: we use maternal grandfather/mother to refer to our mother's father/mother and paternal grandfather/mother to refer to our father's.)
We use 'in law' to refer to those related to us by marriage, such as father-in-law, mother-in-law, son-in-law, daughter-in-law, etc. For example, if Gary and Mary were married, Gary's father, Michael, would be Mary's father-in-law.