User blog comment:B214/Question and Answers 3/@comment-4547267-20140921153656/@comment-73.178.133.185-20140922171811

Good point, Otaku.

You need to understand their relationship. Even after Akeno truly fell in love with Issei, she still pushed Issei to finally confess. Despite her feelings, the fact she was openly targeting him could be seen as her trying to push Rias into acting, otherwise who knew when the two of them would get together. And from the start, she aimed to be a mistress, not the legal wife, meaning that she always wanted the two to be together -- just that she wanted to be with them too. Rias would be able to understand this, at least in part, and thus why they could still remain friends.

There's also the fact that Rias isn't a normal girl. Normal girls wouldn't be able to accept their boyfriends wanting a harem. Remember, Rias is a Gremory, who are known to hold great affection (for their servants).