John Lascelles, the Protestant reformer, stated that his sister telling him about Queen Catherine's promiscuity in her grandmother's household was because of his suggestion that Mary ask for a position as one of the Queen's ladies. How do we know that is true however? He had disdain for Catherine and wanted a more reform-minded monarch.
Perhaps Mary told her brother what she knew of Catherine's lifestyle as a young teenager because she shared his goal of getting a new Queen consort, and knew that he would tell all to Cranmer. I think Mary was probably very straitlaced and didn't approve of Catherine's behavior at the time of her own employment in the Duchess's household. It seems to have been a not well-supervised house to say the least, and probably Mary felt it was for the best that the King was told.
Perhaps Mary told her brother what she knew of Catherine's lifestyle as a young teenager because she shared his goal of getting a new Queen consort, and knew that he would tell all to Cranmer. I think Mary was probably very straitlaced and didn't approve of Catherine's behavior at the time of her own employment in the Duchess's household. It seems to have been a not well-supervised house to say the least, and probably Mary felt it was for the best that the King was told.