Were Adolf Hitler's parents related before their marriage?

Yes they were most certainly related. Am taking WWII & Holocaust class & at university library I viewed a signed & dated request sent to the Bishop of Linz. The Bishop, being a very wise man, refused the dispensation for Alois & Klara to marry. They were related on both sides of their family tree. One branch of the tree was a second degree relation and the other branch was a third degree relation. Alois & Klara sent a request to the Vatican for a Papal dispensation to marry. The Pope granted permission & in my cynical view Herr Hiedler(the original spelling of Hitler) probably made a donation to the Vatican along with his request for dispensation to marry Klara. Adolph was the youngest of six children, Alois had 2 with his previous wife but only Adolph & his sister Paula lived beyond childhood. Paula had little contact with Der Fuhrer & lived under a pseudonym after Adolph's suicide but Paula is the only family member to have the name Hitler on her tombstone. Basically Adolph Hitler is the product of inbreeding and a fine example of what happens when cousins marry cousins. Although he was a notorious hypochondriac and drug abuser maybe some of his physical ailments stemmed from the inbreeding issue.

This is what Wikipedia has to say about it:
Adolf Hitler was born in Braunau am Inn, Austria, the fourth child of six. His father, Alois Hitler, (1837–1903), was a customs official. His mother, Klara Pölzl, (1860–1907), was Alois' third wife. She was also his half-niece, so a papal dispensation had to be obtained for the marriage. Of Alois and Klara's six children, only Adolf and his sister Paula reached adulthood. Hitler's father also had a son, Alois Jr, and a daughter, Angela, by his second wife.

I saw another source that said he was their third child, so take some of the info at your own risk.

They were indeed. Klara Pölzl, Alois Hitler's third wife, was also his half-niece.