User blog comment:B214/Question and Answers 6/@comment-185.112.105.215-20161218170008/@comment-9595671-20161220195126

She can't, not for another 10,000 years atleast due to Vol 21's events due to being sealed with Trihexa. Even then Trihexa's status in terms of strength after they're all out may make it a problem.

If the beast is finally weak enough to be killed (especially if Ophis eventually gets de-nerfed) then they're good, if not... well atleast Issei and Vali are more powerful to handle the monster by then.

Though at the same time, by that point it's likely that Rias will have already figured it out. She and Sirzechs got their power from their mom so it's likely she'll be the one that teaches Rias instead... problem is her parents embarrass her ^^;