My cat veronica had another batch of kittens and we gave away all of them exept for one

and that one is is always afraid of people ever since it was bored it scratches all the time if you walk by but if my little baby sister walks by it doesn't scratch her she can pick it up but when i try to take it away from her it claws away from me i haven't done anything to it except feed it so why is it so afraid of people and how can i get it to not be afraid of people