if it was 1 in 50 then yeh id say go the amnio, but 1 in 300 is only a small chance.
Would the results of the amnio make your sister change her mind about the pregnancy (i know thats a horrible question, but one that needs to be asked) if the answer is no, then i definetly wouldnt get it done, and at the 20 week scan there'll be markers for downs should the baby have it, and if it does, she can reseaarch all she's going to need to know about the syndrome, and if there are no markers, then she can continue till 40 odd weeks without having to worry.
Google search Kellie hamptons blog, she had a DS bub and didnt know beforehand, its a very heartwarming blog about her beautiful little girl