  Recent experimental results [A. Rusydi et al., Phys. Rev. Lett. 97, 016403
(2006)] revealed the stability of a peculiar charge density wave (CDW) with odd
period (three or five) in the coupled Cu$_2$O$_5$ spin ladders in
Sr$_{14-x}$Ca$_x$Cu$_{24}$O$_{41}$, while the CDW with even period is unstable.
Here we resolve this contradiction by solving the effective t-J model for the
two coupled chains supplemented by the intersite interaction term V. Crucial to
this finding is the novel interaction term V (responsible for the interladder
repulsion between nearest neighbor holes), which is derived from the on-site
Hubbard-like repulsion between electrons on the same oxygen sites but within
two distinct orbitals which belong to two neighboring Cu$_2$O$_5$ ladders in
the charge transfer model.
