About cd

Is it true we can use cd only to go to child or grandchild directories but not sister directories?

Who says we can’t? If i am correct you could use:

cd ../sisterdirectory

cd … will go to parent directory, and then from there, go to sisterdirectory.
We can also do:

cd /path/to/sisterdirectory

which will go from the root directory (the directory that holds everything), or we can use ~ for home directory:

cd ~/path/to/sisterdirectory