Get delta of parent form field widget
Found it in $context['form']['#parents']. This is an array that contains the keys of the parent widget. In my case it looked like 

[ 0 =&gt; 'field_pagebuilder',
  1 =&gt; ___PARENT_INDEX___,
  2 =&gt; 'subform',
]


which allowed me to generate my CSS selector for the states API:

$deltaParent = $context['form']['#parents'][1];