Is the food I ate, which my sister brought for my other sister's children, considered unlawful (haram), and what should I do if it is?

We fear that your question stems from overthinking and intrusive thoughts (waswasah), for such minor things that your sister brought to your house are customarily tolerated among relatives. Customary permission takes the place of actual permission, so there is no issue with what you ate of the sweets. This is unless you predominantly believe that your sister would not approve of such an act, in which case you must seek her forgiveness (istihlal) or pay her back for what you ate.