Is the questioner sinful for burying his cousin's decomposed body, which could not be washed, without sprinkling water on it, due to his unawareness of that at the time?

Washing the deceased is a communal obligation (fard kifaya) and a matter of consensus (ijma'), and it is a right of a Muslim upon another Muslim. If it is possible to wash the body, then it is obligatory. If there is a fear that the body will disintegrate by washing, then water should be poured over it. If there is a fear that it will disintegrate by pouring water, then tayammum (dry ablution) should be performed. If it is impossible to wash part of the body but not another, then the part that can be washed should be washed, and tayammum should be performed for the remainder. If the time for washing has passed due to ignorance of the religious ruling, then what is obligatory is to seek forgiveness (istighfar) and to pay attention to the necessity of investigating the religious ruling before acting.