Is there a sin upon a woman whose husband has passed away if she uncovers her face in front of his brothers when bidding him farewell? Does this affect the validity of her waiting period ('iddah)? And does the deceased know anything of what is happening around him after death?

The waiting period (iddah) for your deceased husband ends on the 28th of Safar, completing the four months and ten days prescribed for a widow.

As for your husband's brothers seeing your face, it does not affect the iddah, and you bear no sin if it was not intentional, for the Messenger of Allah, peace and blessings be upon him, said: "Allah has forgiven my Ummah for their mistakes, forgetfulness, and what they are forced to do."