In "Assign patient to the family" modal move the Family name field right next to "Add to an existing family" button. The current position implies the Family name needs to be entered even if user wants to create a new one.
Also, when "Add to an existing family" button is chosen, it would be nice to move the focus to Family name field.
Right now user can tab to this field or click on it, which triggers "Add to an existing family" option to be automatically selected. I am not sure if this will cause some confusion with users ( e.g. they clicked on the field without realizing that they are now searching for existing families and not adding a new family).