User blog comment:B214/New anime 2017 discussion blog/@comment-97.86.117.132-20161230234256/@comment-201.239.133.207-20161231174849

in the story it was said that Rias did not become head of the house of Gremory, but it isn't clear if she rejected to position from the beginning or she stepped down later in favor of Milicas, because the later could pretty much explain why she didn't have Issei's first child, if both parents were to be too busy (we know Issei is extremely busy in the future, having time only for work and kicking ass), it was better for both to wait and have at least his mother raising him.