They are related but they are in different genera according to the botanical powers that be. What can you do! I must confess that I haven't kept up with the nuts and bolts of their classification. The dawn redwood, Metasequoia, is deciduous, unlike the other two redwoods. That's not unknown but something of an exception among conifers. Larches and the swamp cypress (Taxodium) also drop their leaves in autumn. Whether you're deciduous or evergreen isn't necessarily a big thing classification-wise. There are both deciduous and evergreen species of oak, all within the genus Quercus.