How can I create a relative symlink given a relative path?

Try

parent_path=$(echo "$current_path"/ | sed -e "s|[^/]||g" -e "s|/|../|g")
cd "${current_path}" ; ln -s "${parent_path}${parent_file_to_link}"

This works simply by counting the slashes in "${current_path}".  The desired depth is one more than the number of slashes (e.g., the depth of test/dir/hello, which contains two slashes, is 3), so we simply add a slash: echo "$current_path"/.  Pipe it into sed.  Since we’re manipulating slashes, it’s easier to use a character other than / as the delimiter for sed’s s command; I like to use the vertical bar (|).  s|[^/]||g finds all characters that are not slash and replaces them with nothing.  In other words, it deletes all characters except for the slashes.  So, for the "${current_path}" value of test/dir/hello, we echo’d test/dir/hello/ and then chopped that down to ///.  Then s|/|../|g" changes every / into ../, so we end up with ../../../.

Note: this assumes that "${current_path}" does not have any excess (unnecessary) slashes in it.  For example, test/dir//hello and test/dir/hello/ are logically equivalent to test/dir/hello, but they contain a misleading number of slash characters, which will corrupt this process.

P.S. Always quote all shell variables unless you have a reason not to and you’re sure you know what you’re doing.  Using braces (as in ${variable_name}) is not the equivalent to quoting.

something like this should work

file="/var/log/dmesg"
ln -s $file $(dirname $file)/../

Assuming you want to create a:

/home/kostas/test/dir/hello/link -> /home/kostas/file/to/link

symlink (assuming /home/kostas is your current directory), but using a relative link, that is:

/home/kostas/test/dir/hello/link -> ../../../file/to/link

Then you could do (with GNU ln):

$ current_path=test/dir/hello
$ parent_file_to_lunk=file/to/link
$ ln -rsvt "$current_path/" "$parent_file_to_lunk"
‘test/dir/hello/link’ -> ‘../../../file/to/link’

Without GNU ln, but with GNU bash -O extglob or zsh -o kshglob or ksh93:

$ ln -s "${current_path//+([^\/])/..}/$parent_file_to_lunk" "$current_path/"

(note that it assumes that the components of $current_path are not symlinks themselves)