How to include a category in Recent Posts Widget

I am using the Velux theme for my site, https://replacementchildforum.com. If you visit any of the pages with a sidebar, you will see the Recent Posts widget. Currently, that widget is only displaying the entries on the Blog tab. The Category “Letters” is part of the blog and has it’s own menu itrem but the Letters is not being included in the Recent Posts widget.

Currently, I am listing them manually in a Text widget to simulate the recent Posts widget but would like to incorporate the category Letters entry into the recent posts.

Is there some css or another way to include the Letters in the Recent Posts widget automatically?

Thank you.

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