I'm attempting to create a listing of brother or sister pages (not posts) in WordPress to populate a page's sidebar. The code I have written effectively returns a page's parent's title.

<?php
$parent_title = get_the_title($post->post_parent);
echo $parent_title; ?>

When I comprehend it, you'll need a page's id (instead of title) to retrieve a page's brothers and sisters (via wp_list_pages). How do i obtain the page's parent's id?

Alternate approaches are welcome. The aim would be to list a page's brothers and sisters, not always just locating the parent's id.

$post->post_parent is providing you with parents ID, $post->ID provides you with the present page ID. So, the next can place a page's brothers and sisters:

wp_list_pages(array(
    'child_of' => $post->post_parent,
    'exclude' => $post->ID
))
<?php if($post->post_parent): ?>
<?php $children = wp_list_pages('title_li=&child_of='.$post->post_parent.'&echo=0'); ?>
<?php else: ?>
<?php $children = wp_list_pages('title_li=&child_of='.$post->ID.'&echo=0'); ?>
<?php endif; ?>
<?php if ($children) { ?>
<ul class="subpage-list">
<?php echo $children; ?>
</ul>
<?php } ?>

Avoid using the exclude parameter, just target that .current_page_item to distinguish.