I have to agree, it does sound rather youthful and energetic!
Of the combos you provided I think my favorites are
Lennox Alexandra and
Lennox Genevieve. With
Alexandra, I love the repeated 'le' and 'x' in the middle.
Genevieve seems to fit the style of the name really well and I could see them actually being used together.
Great sibling names! I couldn't think of a sister either. :)
Thank you!
http://babynames.com/namelist/9863712