User blog comment:SpookyBoogie27/Light Novel Spoilers/@comment-82.178.115.57-20140207040435/@comment-1359322-20140207063809

If Ishibumi intended to do that, he would have done that already, he had more than plenty of time to do it. Plus Sona already mentioned, she would only someone smarter than herself. Issei already failed badly in that criteria.

Then there's also the issue with Issei and Sona. Issei isn't showing any interest one Sona because of his friendship with Saji, while Sona feels jealous of Issei because Issei did what she couldn't do which is to help solve Rias' problem.