Naw, oyakodon is bad.
Buuut, since you mention it, Tatsu's Dad seems well enough to support another wife. But Polygamy is a crime in America and Japan. Ironically adultery is not a crime in either country, but at least Japan had civil offense measures regarding adultery.
And even then, it would be kinda weird for your dad to marry your mother in law. But i don't know of religious texts prohibiting that practice specifically.