Nested include directives fail due to relative pathing issues

[ttutko]

Describe the bug

With the following project structure:

└── source  
        ├── _static  
        ├── _templates  
        ├── conf.py  
        ├── index.rst  
        └── parent  
            ├── child  
            │   ├── mychild.rst  
            │   └── mysibling.rst  
            └── myparent.rst

Where myparent.rst has the following include directive:
.. include:: ./child/mychild.rst

and mychild.rst has the following include directive:
..include:: ./mysibling.rst

The html build will fail with:

source/parent/child/mychild.rst:3: CRITICAL: Problems with "include" directive path:
InputError: [Errno 2] No such file or directory: 'source/parent/mysibling.rst'.

Per issue #6805, this sort of makes sense as "mychild.rst" gets included in "myparent.rst" making everything then relative to "myparent.rst". However, if I change the include in "mychild.rst" to then be:
.. include:: ./child/mysibling.rst

The following error occurs where you will notice that the sub directory is repeated an extra time:

/docs/src/source/parent/child/mychild.rst:3: CRITICAL: Problems with "include" directive path:
InputError: [Errno 2] No such file or directory: 'source/parent/child/child/mysibling.rst'.

This leads me with no clear path on how to do this and doesn't make sense to me.

How to Reproduce

I've placed a minimal sample on a Github repository: Sphinx Include Bug.

Environment Information

--bug-report doesn't appear to work anymore but this is with version 5.3.0

Sphinx extensions

No response

Additional context

No response