IOTL Sigismund married Barbara Zapolya because he wanted to counterbalance Habsburg influence on the court of Vladislaus (thus he needed to strenghten position of Janos Zapolya, who was leader of anti-Habsburg 'national' party.) If Sigismund himself is King of Hungary there is no need for such action. Also, Janos Zapolya has not married early IOTL, because he wanted to marry Vladislaus' daughter (but was refused). Now, Anna is not daughter but niece of King of Hungary, thus it would more sense for Janos to marry someone more closely related to Sigismund (perhaps his youngest sister, if Sigismund still sees need for such match, Janos could also marry Margaret Hohenzollern, Sigismund's other niece). Marrying his own subject would not be in Sigismund's style (knowing how he reacted to second marriage of his son IOTL)