Complex sentence: René de Cossé married into the Gouffier family, just then very powerful at court, and became premier panetier (chief breadmaster) to Louis XII. Two of his sons were marshals of France.
Split into simpler sentences: René de Cossé married into the Gouffier family, which was at that time very powerful at court. He was awarded the title of premier panetier (baker extraordinaire) to Louis XII. Two of his sons were marshals of France.