So basically when I read Qur'an I did not do istijna and did wudhu. Is it a sin. I knew I did not to it but thought istijna is not needed for wuhdu but needed for prayer
Istinja' is part of the self purification. It is needed when there's najasah on your Anus either due to wind or poop. In the given cases your taharah will not be completed without doing it and therefore your wudu' would be invalid. Else it is in best case optional.
Taharah is a pre requirement for reading in the moshaf.