Complex sentence: Rubio did not mention in the letter that Cicilia is his brother-in-law, failing to disclose the relationship in the letter is not illegal, but can be construed as a conflict of interest.
Split into simpler sentences: Rubio did not mention in the letter that Cicilia is his brother-in-law. Failing to disclose the relationship in the letter is not illegal, but can be construed as a conflict of interest.