Let's denote your age as x. Your sister is then (x + 1) years old, and your mum is 3 times older than your sister, so she is (3 * (x + 1)) years old.
Now, you mentioned you are the youngest at 12 years old. So, x = 12.
Your sister is (12 + 1) = 13 years old, and your mum is 3 * (13) = 39 years old.
The range of ages in Elena's family is from 12 to 39 years.