User blog comment:B214/Question and Answers 2/@comment-4547267-20140319005102/@comment-24468758-20140319064527

They are engaged it's just that it is still only known by the Gremorys and the ones closely affiliated with them (but Issei is also still oblivious to this). In Vol.9 Bael Agares said he is the future brother-in-law of Sirzechs and in the quotes of Vol.17 Sirzechs calls him his brother in front of Euclid.