。
She made her younger brother walk the dog.
Literal
She [topic-は] younger-brother [to-に] dog [possessive-の] walk [object-を] made-do.
犬の散歩をさせた uses the causative form of する — させた = 'made (him) do.' The に marks the person being made to act (弟). This is straightforward causative: an older sibling directing a younger one to handle the dog walk. Whether it's a command or a delegated chore depends on context — the causative itself is neutral.