Final answer:
Colonel Fitzwilliam, Mr. Darcy's cousin, joins him at Rosings in 'Pride and Prejudice.' His presence at Rosings Park reveals crucial information about Darcy's character and actions to Elizabeth Bennet.
Step-by-step explanation:
The family member who joins Mr. Darcy at Rosings in Jane Austen's novel Pride and Prejudice is his cousin, Colonel Fitzwilliam. Colonel Fitzwilliam is a pleasant and amiable character, contrasting with Mr. Darcy's initially proud and distant disposition. The visit to Rosings Park, Lady Catherine de Bourgh's estate, provides pivotal moments in the development of the relationship between Elizabeth Bennet and Mr. Darcy. During the stay, Elizabeth becomes privy to Darcy's feelings, and it is also when she learns from Colonel Fitzwilliam that Darcy may have influenced Bingley's decision to leave Netherfield, thus complicating her feelings for him.