It looked a little hesitant to me as I said in Comments - I don't know (this sig). It has 30 days return policy and hopefully it was paid for with PayPal. ;) Jennifer will be fine either way in that case.
But here is a nicer copy from the same year 1951 with unfaded artistic dustjacket for $100 with a DJ and no fake sig...why not return? If that is not his sisters proxy then it's a "real forgery" and damaged/worth-less. Here is a link...