Quite recently, ATK_RELATION_NODE_PARENT_OF was implemented in atk and at-spi. This is a reciprocal relation to ATK_RELATION_NODE_CHILD_OF. It should be carried over into the new atk and at-spi.
This has been fixed.
Support for NODE_PARENT_OF was added in commit 9016e554a3e30038f95cb21 of at-spi2-atk.