Explanation:

Solution: 
When Kaira was born:
Mother was 30.
She is 4 years younger to her brother, so brother was 4 years old.
Sister was 4 years old when brother was born, so sister is 4 years elder to brother, so sister was 8 years old.
Father was 30 when sister was born, so father is 30 years elder to sister, so father was 30+8 = 38 years old.
Now when Kaira was born, mother was 30 and father was 38
So difference in their ages is 8 years. So when mother was born, father was 8.