I won't go to the party unless Jane is invited,either.为什么用ei

I won't go to the party unless Jane is invited,either.为什么用either?我的猜测有两种：一种是主句否定,后面也要否定；一种是unless= if not ,和never,hardly 一样是代表否定的词.问了两个老师,也都各回答了一种,所以提问,希望能有个•明确•的答案.