There is no sin attached to the performance of an Upanayana ceremony instead it is one of the essential samskars that need to be undergone by Hindus. The performance of this ritual signifies the coming of age of a child to begin his education as a Brahmachari.
As mentioned in The Grihya Sutras while talking about the Upanayana ceremony's place in the Brahmanas (Page 7):
At the end Saukeya, filled with astonishment at the wisdom of Uddalaka, declares that he wishes to come to him as a pupil (upayani bhagavantam), and Uddalaka accepts him as his pupil. It is the telling of this story and the decisive words upayani and upaninye which furnish the occasion for introducing the following section on the Upanayana.
If we check the corresponding text in the Shatapatha Brahmana 11.5.3 it corroborates this claim and also adds another perspective:
13 Saukeya, thus instructed, said, 'Here are logs for fuel: I will become thy pupil, reverend sir.'
He replied, 'If thou hadst not spoken thus, thy head would have flown off : come, enter as my pupil!'
'So be it,' he said. He then initiated him, and taught him that pain-conquering utterance, Truth: therefore let man speak naught but truth
It would appear that if Saukeya had not urged Uddalak to become his pupil and initiate Upanayana that would have been a much bigger sin so much so that his 'head would have flown/fallen off.'
The next section 11.5.4 actually details the progress of the ceremony with the following words:
1 He says, 'I have come for Brahmakarya:' he thereby reports himself to the Brahman. He says, 'Let me be a Brahmakârin (student):' he thereby makes himself over to the Brahman. He (the teacher) then says, 'What (ka) is thy name?'--now Ka is Pragâpati: he thus initiates him after making him one belonging to Pragâpati.
The ritual that helps turn someone into a student of Vedas can not be sinful.