User blog comment:B214/Talk About What You Want! 2/@comment-108.27.243.235-20140707032420/@comment-1359322-20140707110809

Even so, Riser has never stated any intention to make his servants his mistresses.

If there's a true reason why Rias can accept Issei but not Riser, it's because of the time they spend together and exposure to each other.

Compared to Riser, Issei has spent more time with Rias, so Rias has more time to understand Issei and Issei has showed the side where he truly cares for his friends. On the other hand, she rarely spent time to learn about Riser to see how Riser truly is as an individual. No one knew that Riser was a caring brother before right? Not even Rias or the others.

In the end, it's just the situational thing. Issei had the advantage on that part while Riser didn't. Of course, it's not bad, after all like Lord Phenex said, Riser needed a taste of defeat so that he can progress more.