How old or big is your little sister? There are a several ways to cure Goblin of this problem, but it depends on whether or not your sister is big enough to defend herself.

If she is, having her lift her knee when Goblin jumps will make her look bigger and more threatening to him. After all, Goblin does not want a big, relatively pointy object in his chest. If she is even bigger, she can push the dog off as he is jumping.

If she is too small to do either, you can try the can of coins. Fill a soda can about quarter full of pennies and/or nickels and tape the hole shut. Keep the can with you and keep an eye on Goblin. When he starts to jump on anyone, shake the can to make a loud noise. After about a week, he should associate the loud noise with his urge to jump up on your sister and anyone else for that matter. He shouldn't want to jump anymore.