Search only siblings of current file?

[mikabytes]

Love CtrlP for all my projects. Noticed quite often I'm changing to a sibling file of current file/buffer, and have been trying to write a script to enter such a search mode.

nmap <Leader>a :CtrlP $(expand('%:p:h'))<CR>

Things are not quite working as I expected though. The path somehow seems to be remembered for the next time when I want to make a global search.

Any ideas?