'Read more' in sidebalk

assuming the code in the sidebar is using the function the_content() then this will be default only stop at the ‘read-more’ if on multi-post pages, such as index or archive.

on a single post, it will show thwe full content (regardless of sidebar).

http://codex.wordpress.org/Customizing_the_Read_More
http://codex.wordpress.org/Customizing_the_Read_More#How_to_use_Read_More_in_Pages

to keep the short form, you need to add:

<?php global $more;
$temp_more = $more;
$more = 0; ?>

before the code with the_content()
and this after:
<?php $more = $temp_more; ?>

how and where depends on the code for the widget/section of the ‘news-item’ area.