Is a person's disclosure of something they intend to embark on (such as postgraduate studies) to some close relatives (like a mother or brother) or to someone in whose presence they mention a part of it, considered a violation of the Sunnah that calls for keeping matters confidential?

Consider the benefit in concealing needs. If concealment will lead to a deterioration of relations, and disclosure is preferred to avoid that, then mending relations is more приоритетная; because the command to conceal is for guidance, not obligation, and this is assuming the authenticity of the hadith (definition) mentioned in that regard. A person should consider the benefit and conceal or disclose as the benefit dictates.