Complex sentence: The royal couple turned their attention to Anna's younger unmarried sister Elisabeth, they wished to have her married but any suitors that came forward Elisabeth refused, then in 1310 Elisabeth married John of Luxembourg, this caused Anna and Elisabeth to fall out.
Split into simpler sentences: The royal couple turned their attention to Anna's younger unmarried sister Elisabeth, they wished to have her married but any suitors that came forward Elisabeth refused. Then in 1310 Elisabeth married John of Luxembourg, this caused Anna and Elisabeth to fall out.