#layout.html.twig
{{ show }}

#base1.html.twig
{% extends 'layout.html.twig' %}
{% set show = 0 %}

#base2.html.twig
{% extends 'base1.html.twig' %}
{% set show = 1 %}

#base2.html.twig
{% extends 'base1.html.twig' %}
{% block content %}
    {% set show = 1 %}
    {{ parent() }}
    {{ show }}
{% endblock %}

#layout.html.twig
{% block content %}
    {% set show = 0 %}
    {{ show }}
{% endblock %}

#base1.html.twig
{% extends 'layout.html.twig' %}
{% block content %}
    {{ parent() }}
    {% set show = 1 %}
    {{ show }}
{% endblock %}

#base2.html.twig
{% extends 'base1.html.twig' %}
{% block content %}
    {{ parent() }}
    {% set show = 2 %}
    {{ show }}
{% endblock %}