WordPress has support for threaded comments, but you have to enable it yourself, by adding a script to your head. You can use WordPress built in function wp_enqueue_script() to load the script, by adding this snippet to your functions.php
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When the script is loaded, you have to activate the threaded comments in the settings tab in WordPress admin, under Settings > Discussion. Click Enable threaded (nested) comments and choose the level of nested comments you want.
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