Step one: Ignore the dog on purpose. The reason you need to ignore him is because sometimes small, fearful dogs mistake a giant human's efforts to interact as threatening. Even simple things like eye contact or stooping to pet them can be scary. Do this until he stops barking. This part is key.

Step two: Cut up a piece of sandwich meat into tiny pieces or buy some of those ham cubes. You can also use cat treats. Put them in a bag your pocket. When you go over there completely ignore the dog until the barking stops but then randomly throw the treats on the ground. Each time you throw one, throw it a little closer to you while you are sitting. Praise the dog using it's name in a happy voice when it gets the treat. Keep doing this until you can lay the food on your hand and it will approach and take it. If it starts barking again, stop throwing the treats and repeat. It will help if your sister avoids treating him at all during this process.

Now you are "the kinda cool human who never tries to grab me and always has awesome treats".

Step three: Once the dog is comfortable taking the food from your hand have your sister hand you the dog rather than try to reach out and grab him or leash him for you. Walk around with him a bit, holding him or on a leash while talking nicely to him and giving him treats. If he's small enough, do both! Make sure to rub him so he gets used to you touching him. Then put him down and ignore him again.

Now you are the "the really awesome human who never tries to grab me, gives me awesome treats and even treats me kindly when master demands I am held."

Step four: Have your sister hand you the dog while she leaves the home for a few hours. Make sure he is really hungry so it's best if she hasn't fed him yet. Feed him treats, play with him, etc. Do so while sitting or even laying on the floor. He should be comfortable at this point to willingly climb in your lap. If he does just pet him a bit. When you feel he is completely relaxed try lifting him up for a cuddle on your chest. If he stiffens up just put him back down on your lap and try again another day. After a few times of being his sole caretaker, he should see you as "my other master".