Based on the names Arthur, Oliver, and Sonny, here are some sibling name suggestions, categorized for different approaches:

Option 1: Maintaining a Similar Style (Classic & Slightly Uncommon):

• For a sister: Clara, Theodora, Genevieve, Beatrice, Eloise. These names share a similar slightly old-fashioned yet elegant feel with Arthur and Oliver.

• For another brother: Frederick, Theodore, Edmund, Jasper, Reginald. These names maintain the classic, slightly less common feel of the original names.

Option 2: Modernizing the Style:

• For a sister: Willow, Hazel, Indigo, Skye, Juniper. These names offer a more modern, nature-inspired contrast to the slightly more traditional feel of the original names.

• For another brother: Asher, Ethan, Caleb, Owen, Leo. These names are modern and popular, offering a different vibe while still being relatively easy to pronounce and remember.

Option 3: Focusing on Sound and Rhythm:

• For a sister: Amelia, Cecilia, Eleanor, Sophia, Audrey. These names have a similar rhythmic flow and sound to Arthur and Oliver, creating a harmonious set.

• For another brother: Samuel, Daniel, Gabriel, Nathaniel, Elias. These names share similar phonetic qualities with the original names, creating a pleasing auditory experience.

Option 4: Considering Initial Letters:

If you prefer names starting with specific letters, please specify. For example, if you want names starting with "A" for a sister and "J" for a brother, I can provide more tailored suggestions.

Option 5: Thematic Approach:

If you have a specific theme in mind (e.g., nature, mythology, strong characters), please let me know. This will help me generate more relevant suggestions. For instance, if you like the slightly old-fashioned feel, we could explore names from specific historical periods.

Important Note: The best sibling names depend heavily on personal preference. These are just suggestions to spark your creativity. Consider the flow of the names together, their meanings, and how they sound when said aloud as a group.