To open a file relative to the current one in Helix, you can to the do the following with 24.3:

:o <C-r>%<C-w>

Here, the Control-R allows you to select a register and the special register “%” contains the current file path and inserts into the command line, while the final Control-w chops off the last part of the file name leaving with you current directory.

This is like :o %:h from Vim/Neovim