Explanation:

- Option 1: Youngest brother – In adelphic polyandry, this is not typically the case, as responsibility generally does not fall specifically on the youngest sibling.

- Option 2: Maternal uncle – This option is incorrect as maternal uncles aren't traditionally responsible for the offspring in this marriage structure.

- Option 3: Eldest brother – In adelphic polyandry, often practiced in some cultures like among the Tibetans, children are considered the offspring of the eldest brother, who holds primary responsibility.

- Option 4: None – The responsibilities indeed align with one of the brothers, so this option is not applicable.

Option 3: Eldest brother is the correct answer.