Complex sentence: During his time in Vienna she kept her household in the palais Kaunitz and it was at this time that Dorothea began to play a major part in Charles-Maurice's life, accompanying him to the congress of Vienna and probably becoming his mistress sometime after 1815.
Split into simpler sentences: During his time in Vienna she kept her household in the palais Kaunitz. It was at this time that Dorothea played a major part in Charles-Maurice's life, accompanying him to the congress of Vienna and probably becoming his mistress sometime after 1815.