User blog comment:B214/Question and Answers 2/@comment-4547267-20140402005017/@comment-9339850-20140402043035

Honestly, i would've assumed Rias would've (or should've) been more shocked at Akeno falling in love with Issei, since they are "true" best friends. I feel like Rias and Sona's friendship/relationship is basically because they were both born into prestigous families and were more or less made to mingle, showing proper social manners. While on the other hand Rias and Akeno's friendship seemed more or the normal friendship that flourished into being best friends.

So if Sona feel in love with Issei, I think Rias would feel ticked off at her because she is basically a friend of convenience (because they are now both heiresses of their respective clans) who is interested in her boyfriend/future-soon-to-be-husband.