If Joseph is from the line of David, but was not the true/blood father of Jesus, how did Jesus fulfill this prophecy? Was Mary also from the house of David?
Yes. Luke 3 gives the genealogy of Jesus goes backwards in time, from Joseph (Jesus' earthly "father") to Adam (including Abraham and David). And in Matthew 1, the genealogy of Jesus goes forward in time from Abraham to Mary (including David). So David is on both sides!
I guess I could have just said "yes".
-paid by the word