Is it a sin if one does not fast before receiving the Eucharist?
The essence of sin is not the mere breaking of a law but in the attitude that leads to the breaking of the law (Mark 9:15).
The eucharistic fast is in place to help us appreciate what we are receiving in the Blessed Sacrament. Our body and soul are united, thus this simple fast helps us to hunger for the Bread of Life.
This fast is not intrinsically required for the reception of Communion. The sick and elderly are exempted, and priests are not required to fast after their first Mass of the day.
So someone who is required to fast but does not do so has not done something that is intrinsically wrong. Such a person has violated a legal requirement. Thus their culpability would be based on the reasons for not fasting.
Was the person simply caught in unfortunate circumstances and honestly miscalculated the timing? Such a person certainly is not morally culpable.
Was the person forgetful about the timing due to a lax attitude towards receiving the Eucharist and the laws of the Church? Such a person surely bears some culpability.
Was the person flippantly disdainful of the proper respect for the sacrament and the laws of the Church? Such a person certainly is morally culpable.