How to open branch object equivalent of -r ancestor::parent

[Alexander Belchenko]

To fix the bug https://bugs.launchpad.net/bzr-explorer/+bug/956268
I need to open ancestor related to parent branch instead of tip of 
parent branch itself.

Current code does today:

old_branch = mod_branch.Branch.open_containing(branch.get_parent())[0]

Which is equivalent of -r branch::parent

I need this to be changed to be equivalent -r ancestor::parent

Any suggestions or code snippets will help.