There's Really No Fighting Kyouko-san

Iirc, marriage between family members is illegal if it's not second cousin or further in Japan.

EDIT: After a quick Google search, according to article 734 of marriage law in Japan, quote, 'Lineal relatives by blood, collateral relatives within the third degree of kinship by blood, may not marry, except between an adopted child and their collateral relatives by blood through adoption. Lineal relatives by affinity may not marry,' end quote.

So from what I'm understanding of this with my little understanding of law, the only way these two could have gotten married was if obaa-san was married to a direct-blood relative of her nephew, but wasn't blood herself, and had divorced from that direct blood relative, and either waited 6 months, and then got married, or was impregnated before the divorce by her nephew, divorced the relative, married the nephew during the pregnancy, and then gave birth to the child, according to other parts of Japanese marriage law that I read during the same Google search.

That being said, since this is INCEST and not inseki, like fakku keeps fucking up the tag as, they literally cannot ever marry.