Answer:
Here is the JAVA program:
Sibling.java class:
import java.util.Scanner;  //to accept input from user
public class Sibling {  //class name
//private data members of class
    private String name;  // String type variable to hold the name of sibling
    private int age;  // int type variable to hold the age of sibling
    private int weight;  // int type variable to hold the weight of sibling
    public Sibling() {}     //constructor of class Sibling
    public Sibling (String n, int a, int w)  {  //parameterized constructor
        name = n;  // refers to name field
        age = a;  // refers to age field
        weight = w;    }   // refers to weight field
    public String getName ()    {   //accessor method to get the current name of sibling
        return name;    }   //returns the name field of the sibling
    public int getAge ()    {   //accessor method to get the current age of sibling
        return age;    }   //returns the age field of the sibling
    public int getWeight (){ //accessor method to get the current weight of sibling
     return weight;    } //returns the weight field of the sibling
    public void getInput() {    //to take input name,age and weight from user
  Scanner input = new Scanner(System.in);  //Scanner class object
  System.out.print("Enter the name:  ");  //prompts user to enter the name of sibling
  name = input.nextLine();  //scans and reads the input name from user
  System.out.print("Enter the age:  ");  //prompts user to enter the age of sibling
  age = input.nextInt();   //scans and reads the age from user
  System.out.print("Enter the weight:  ");  //prompts user to enter the weight of sibling
  weight = input.nextInt(); } }   //scans and reads the input weight from user
Explanation:
Here is the TestSibling class that contains main method:
public class TestSibling {   //class name
public static void main (String[] args) {   //main method
String name;  // to hold name of sibling
int age, weight;   // to hold age and weight of sibling
Sibling sib1, sib2, sib3;   // objects of Sibling class
sib1 = new Sibling ();  // creates object of class Sibling for sibling 1 and calls constructor
sib1.getInput();   // calls getInput method using sib1 object to get the name, age and weight of sibling 1
sib2 = new Sibling ();  // creates object of class Sibling for sibling 2 and calls constructor
sib2.getInput();   //calls getInput method using sib2 object to get the name, age and weight of sibling 2
sib3 = new Sibling ();  //creates object of class Sibling for sibling 3 and calls constructor
sib3.getInput();   //calls getInput method using sib3 object to get the name, age and weight of sibling 3
Sibling youngest=null, lightest=null;    //holds the youngest age and lightest weight of siblings
if (sib1.getAge( )<= sib2.getAge( ) && sib1.getAge( ) <= sib3.getAge( ) )   /*if condition checks if age of sibling 1 is less than or equals to that of sibling 2 and sibling 3,  using object of each sibling and getAge method to access age. the logical operator && AND is used so that if condition evaluates to true if sib1 is younger than both sib2 and sib3 */
{ youngest=sib1;}   //if the above condition is true then sets sib1 as youngest
else if (sib2.getAge( ) <= sib1.getAge( ) && sib2.getAge( ) <= sib3.getAge( ) )   // else if condition checks if age of sibling 2 is less than or equals to that of sibling 1 and sibling 3
{youngest=sib2;}   // if above condition is true then sets sib2 as the youngest
else   //if none of the above condition is true then this means that the third has the lowest age 
{            youngest=sib3;    }    //sets sib3 as the youngest
if (sib1.getWeight( ) <= sib2.getWeight( ) && sib1.getWeight( ) <= sib3.getWeight( ) )   //if condition checks if weight of sibling 1 is less than or equals to that of sibling 2 and sibling 3,  using object of each sibling and getAge method to access weight of each sibling
           { lightest=sib1; }   //if the above condition is true then sets sib1 as having the lightest weight
else if (sib2.getWeight( ) <= sib1.getWeight( ) && sib2.getWeight( ) <= sib3.getWeight( ) )  // else if condition checks if weight of sibling 2 is less than or equals to that of sibling 1 and sibling 3
{lightest=sib2; }  //if the above condition is true then sets sib2 as the lightest
else  //if none of the above condition is true then this means that the third has the lightest weight
{ lightest=sib3;   }  //sets sib3 as the lightest
System.out.println("The lightest sibling: " + lightest.getName() +" " + lightest.getAge()+" "+ lightest.getWeight());  } } //calls the getName() getAge() and getWeight() method using object lightest to print the lightest of the siblings
System.out.println("The youngest sibling: " + youngest.getName() +" " + youngest.getAge()+" "+ youngest.getWeight());  //calls the getName() getAge() and getWeight() method using object youngest to print the youngest of the siblings
The screenshot of the output is attached.