Have Sanders Jewish polish parents immigrate to Britain not America. This is the same story with Ed Milibrand.
He wins the 1991 general election after years of Thatchar
Then his Father (the Polish Jewish Immigrant) would have never met his Mother (the second generation Russo-Polish Jewish immigrant who was born in America), thus meaning he would have never been born.
And the second half is as likely as Corbyn having won that election.
EDIT: To build on this, let's assume that Sanders is still born in America 1941, and his parents move to the United Kingdom in 1948-1952, for whatever reason. The earliest Sanders could plausibility be elected to Parliament is at the age of 21, so the first election he can actually contest is 1966. Now Bernie doesn't strike me as the kind of man who would go into politics so suddenly, at least in the sense of entering Parliament. So lets say he defers for a few years and enjoys the the White Heat, assuming he's even a Labour member, maybe running in a Council seat. So he runs in 1970. Not a good year for Labour or the Liberals, and as an American Jewish Immigrant, he's not really going to do well anyway. So he tries again in 1974. Maybe he can get in on a sliver, riding on the dissatisfaction of the Tories, cementing himself with a larger majority in the second '74 election.
Or maybe he doesn't get selected until 1979, when a safe seat with a large Jewish population (Hackney North and Stoke Newington) is opened for him. So he enters Parliament now- he retains the seat in 1983, and is able to steadily build his majority in 1987, avoiding deselection like Ernie Roberts. But where does Bernie stand? Does he stand on the Labour left, which would mean any kind of potential is killed in 1985 and the following years, or does he stand on the right, in which he might find a cushy position in Kinnock's cabinet and in the Modernizers, maybe even defect to the SDP. However, he's not yet prime for Leadership. He's been there for only two Parliaments, and assuming his moving to Britain hasn't caused butterflies of gargantuan proportion, Kinnock isn't going to stand down for him, especially if Thatcher is hanging on for 1991, which would be a Labour slam dunk.