User blog comment:B214/High School DxD DX.4 discussion/@comment-24421137-20170720035349/@comment-10330822-20170725130720

I said, besides Rias. Remember, Rias will be the first one Issei marries, for obvious reasons, but she won't be able to give birth until a few years after Akeno does. Akeno has the first-born, Kurenai Himejima.

So, Rias wins in regards that she marries Issei first and probably takes his virginity, but Akeno is the one who gets pregnant before her. This isn't a big surprise, because pureblood devils have a harder time giving birth. So it was probably easier for Akeno and Asia to get pregnant than it was for Rias, even if it was only by a couple of years.

Other than Rias, its pretty much going as birth order goes. Akeno and Asia have the eldest children, who are 18 in EX, and after Rias, they were the next to be proposed to. Xenovia and Irina had 17 year olds in EX, and so did Rose, which means they have children before Rias, Koneko, Kuroka, and Ravel as well, since all of their children are 16 in EX.

I'm assuming Rose will be proposed to in Typhon's battle against Issei, meanwhile Koneko and Kuroka will probably propose to Issei after their mutual fight in the Vali vs. Rias match.

In short, next volume will probably end with Rose, Koneko, and Kuroka all be engaged as well, which means we are still on track for the birth order.