Question: are arthur and molly related?
Answer: Quite interesting. Also, Molly Weasley and Arthur Weasley are both related to Sirius Black distantly. If you see, there is Ignatius Prewett (husband of Lucretia Black) in the family tree, and must be closely related to the Molly's father (the older generation). So, Molly and Sirius are cousins by marriage.