Let us Assume that The Present Age of your Brother is 29
Given : Your Brother is 5 years Older than You
⇒ The Present Age of You is 29 - 5 = 24
Given : Your Sister is 3 years younger than You
⇒ The Present Age of your Sister is 24 - 3 = 21
⇒ Your Sister will be 21 years Old when your Brother is 29