Directly related is not the same as being a direct descendant. I am directly related to my siblings, and my children, not descended from them. I am a direct descendant of the Doctor, however, and of Joseph and Mary.
I would consider a "direct" relationship as including spouses, parents, grandparents, uncles, aunts, children, siblings (including half-sibs), nieces, and nephews.
Under the law, a direct relation also includes mothers-in- law, fathers-in-law, brothers-in-law, sisters-in-law, and stepchildren.