@contrex,
The old
dictum that one's mother need have been Jewish was very likely the kind of pragmatic adjustment to social reality that the ancient Europeans practiced, with their system of "sister-son" fosterage. You may not know to a certainty who the father is, but there's no doubt about the mother. In ancient Keltic and German tribes, it might have been more than your life were worth to inquire too closely into a woman's sexual partners. Among ancient Europeans, a man would see to the education and the welfare of the sons of his sister, even fostering them if necessary, and his estate would devolve upon his sister's sons. After all, he knew that as his sister was the boy's mother, they shared the same blood.
EDIT: He couldn't necessarily be sure his wife's sons were his, and with those big nasty bitches, skilled in the use of spear and sword, you might not want to bring the subject up.