Mr. Brandon is the deceased elder son of Mr. Brandon Senior, making him brother to Colonel Brandon and at least one sister.
Based on the Colonel’s story, Mr. Brandon was likely either as hard-hearted as his father or too cowardly to stand up to him. We are not told how much older he is than his brother, but we know that he had to be older than 17 (though likely not by too much) when his father married him off to his cousin Eliza in order to keep her fortune in the family. He agrees to this marriage with the full knowledge that Eliza is in love with his brother and has been forced into it.
Once married to Eliza, Colonel Brandon says he “had no regard for her; his pleasures were not what they ought to have been, and from the first he treated her unkindly.” Eventually, Mr. Brandon divorced Eliza for adultery while she was pregnant with or just after she gave birth to an illegitimate daughter.
Mr. Brandon was also likely not a good financial manager. Mrs. Jennings says he left the estate at Delaford “sadly involved.”
