Who is the legal guardian for the mother and sister in the event of the eldest son's absence and prolonged travel, given that the younger son resides with them?

Men have general guardianship over close female relatives. This guardianship entails preservation, protection, enjoining good, and forbidding evil. This applies to both you and your brother, depending on the situation, ability, and status. As for guardianship in marriage (wilayat al-tazwij), it belongs to both of you. However, if the sister designates one of you, then the right is his. If she does not designate, then it is recommended to prioritize the one who is superior in knowledge and religiosity, then the elder in age, if they are equal otherwise.