Check Structure for Entry or Child of Entry
The containment operator in does have problems working with whole element models. I'd try and modify your conditional to compare entry IDs instead:

{% if entry.id in craft.entries.section('general').descendantOf(42).ids() %}
    ...
{% endif %}


To include that parent page ID to that array you would do that with the merge filter:

{% set aboutPagesIds = craft.entries.section('general').descendantOf(42).ids() %}
{% set aboutPagesIds = aboutPagesIds|merge([42]) %}