Considering "Harley Jae" as a starting point, here are some sibling name suggestions, categorized for different preferences:

Similar Style (Modern, slightly edgy, with a touch of uniqueness):

• For a Brother: Jaxon, Rhys, Arlo, Jasper, Maverick, Griffin, Ronan, Zane.
• For a Sister: Willow, Indigo, Juniper, Lyra, Scarlett, River, Wren, Gemma.

Complementary Style (Balancing the edginess of Harley with softer elements):

• For a Brother: Ethan, Caleb, Owen, Samuel, Elias, Benjamin, Asher.
• For a Sister: Eleanor, Clara, Olivia, Sophia, Amelia, Charlotte, Grace.

Thematically Linked (Nature-inspired, considering the "Jae" element which could suggest nature):

• For a Brother: Forest, Brooks, River, Stone, Clay.
• For a Sister: Willow, Aspen, Skye, Raina, Autumn.

Initial-Sound Matching (Alliteration or similar sounds):

• For a Brother: Hudson, Harrison, Hunter.
• For a Sister: Jillian, Jocelyn, Juliet.

Considering Name Length and Syllables:

• Shorter Names: Leo, Kai, Ava, Mia.
• Longer Names: Alexander, Sebastian, Victoria, Isabella.

Note: These are just suggestions. The "best" sibling names depend heavily on personal preferences and the overall family dynamic. Consider the flow of the names together when spoken aloud, and ensure they complement each other without being too similar or too disparate.