User blog comment:Ixarising/High School DxD Dream Battle/@comment-71.108.70.155-20140120005532/@comment-24468758-20140128181139

If they would both get serious to kill each other then it would be quite the destructive fight and no one should get near them, since even that would kill most of the beings. The end of that fight can't be detertermined as of now. Even later on it will still be difficult since both getting "dead serious" against each other is impossible. No one would kill his brother-in-law if they have even a little bit of the strong relationship as these two. Sirzechs is a siscon and he is deeply interested in Issei since his first appearance, he cares for him as the future husband of Rias and Milicas admires Issei very much, too. Issei admires Sirzechs strength as seen when he tries to adapt the movement of demonic power to his dragon shots. Moreover he has deep gratitude towards him for giving him the possibility to break off Rias' engagement with Raiser. So in the end would be interesting but we will most likely never get a real answer for this.