How old is your dog? I think it'd be much easier with a pup but it's still possible to work with an adult as well.

Like Karen said, I suggest having him on a leash. Have him sit, praise him like crazy. Then allow one sibling at a time to slowly approach him and talk to him in a soothing manner. It's very important that you don't tighten the leash or act nervous. Dogs will sense when you are uptight and act on those feelings they're getting from you. Keep him sitting and praise. You can even try having your siblings asking him to sit and praising him. Just try having your siblings relatively close to him at random times around the house. He should get use to them and once he acts comfortable being close to them, they could approach him.