When you were 10, you sister was 5 years old. Which means that she is 5 years younger than you. Now that you are 40, your sister is 35.
Good job, are there other riddles like this that you know?
Here are some other logic puzzles or riddles I can solve:

• The more you take, the more you leave behind. What am I? Footsteps.

• Spinning fast, arms outstretched, what am I? A windmill.

• The more of them you have, the less you see. What are they? Darkness.

• What gets broken without being held? A promise.

• What has a head and a tail but no body? A coin.

• What has 88 keys but can't open a door? A piano.

• What has a ring but no finger? A telephone.

• The more you lose, the more you gain. What am I? Weight.

Would you like me to try and solve another riddle or logic puzzle for you?