Given the name Brendan, here are some sibling name suggestions, categorized for different preferences:

Similar Style (Irish/Celtic feel, strong and slightly uncommon):

• Masculine: Keelan, Finnian, Declan, Aidan, Cian, Cormac, Eamon, Liam (slightly more common but fits the style)
• Feminine: Saoirse, Aisling, Róisín, Clíodhna, Caoimhe, Niamh, Eimear

Complementary Style (softer, more modern, or contrasting):

• Masculine: Owen, Ethan, Caleb, Jasper, Arthur, Theodore, Elias
• Feminine: Clara, Eleanor, Willow, Hazel, Audrey, Isla, Olivia

Names with Similar Sounds/Initials:

• Starting with B: Brian, Blake, Bryce (masculine); Bridget, Brooke, Brynn (feminine)
• Similar sounds: Kieran, Adrian (masculine); Erin, Karen (feminine)

Thematic Suggestions (depending on desired theme):

• Nature Theme: River (masculine/feminine), Aspen (feminine), Forest (masculine)
• Strong/Powerful Theme: Magnus (masculine), Athena (feminine), Rex (masculine)
• Literary/Classic Theme: Arthur (masculine), Beatrice (feminine), Henry (masculine)

This list provides a variety of options, considering different styles and approaches to sibling name pairings. The best choice will depend on personal preferences and the desired overall feel for the sibling set. Consider saying the names aloud together to see how they sound as a pair.