Set twig variable to json file as an include
To capture chunks of text it is better to use the {% set var %}/{% endset %} tag. This allows you to assign "larger" amount of data to a variable. It's also possible to pass content from another file to the variable this way in combination with include.
{% set json %} {% include "content.json" %}{% endset %}{{ json }}(sidenote: Content captured as chunk is being treated as safe)