In the USA, I would prefer the spelling
Kieran as
Ciaran will lead to many more mispronunciations when people first see the name.
The only assumption I would make about someone with the name is that his parents had Irish ancestry or were otherwise interested in
Ireland.
I don't think a boy named
Kieran would be teased just because
Kiera is a popular girls' name. The -n ending is so popular for boys' names right now that many people will automatically assume that
Kieran is male and
Kiera is female.
I see no reason why
Kieran would not "age well".
The combination sounds fine to me, and I think
Kiefer and
Kieran are just far enough apart that they would be "doable" for brothers.