Where is the / or /sbin file system or directory mounted from on Android?
The rootfs is setup by kernel and init,  /sbin is part of initrd.img aka ramdisk.
Initrd.img is part of boot partition, and needs to be unpacked to be edited.

Here is some good info on boot process and editing  boot partition http://www.slideshare.net/chrissimmonds/android-bootslides20
XDA forum HOWTO: Unpack, Edit, and Repack Boot Images

Also, newer devices are requiring boot partition to be signed, and unless bootloader is unlocked, any editing will cause device not to boot.