Larry Sanders, Senator Sanders elder brother, is the candidate of the Green Party of England and Wales in the Witney by-election (to fill the vacancy caused by David Cameron resigning his seat in the UK Parliament). The election is on 20 October, so we will see if any of the American brother's celebrity makes any difference. I suspect not, as the Greens will probably come 4th or 5th, in a safely Conservative rural and small town district of central England.
He is not the Sanders this thread is about and the voting is not in a US Presidential election, but it is an electoral test for a Sanders.
http://www.bbc.co.uk/news/uk-politics-37447086