Isolde Neumann was John of Gaunts nurse when he was little and live in Gaunt or Gent which is in the Neterlands. She died in the plague leaving him after she had promised him he would always be with him. Pietre was her son who was jealous of John for he believed that his mother loved John more. She may have looked after him ( John) better as she would have been paid to do so.
Pietere yurns up in London as an adult and learning of Johns unpopularity he wrote 'posters' saying , basically, that John was not the son of the King but a buthchers son brought in to re[lace the prince who had died. As children Pietre had taunted John with this. John found out who was writting these 'posters' and had him 'deported back to Flanders.
Hope that helps.