On the Type-Moon Wikia entry for Geis it says this:
Contract used by Kiritsugu Emiya
Binding magecraft: Target – Kiritsugu Emiya
The Crests of the Emiya family hereby command: Provided that the following conditions are met, this oath shall become a commandment and bind the target without exception.
To the fifth head of the Emiya house, Kiritsugu, son of Noritaka: regarding both Kayneth Archibald El-Melloi and Sola-Ui Nuada-Re Sophia-Ri, all intentions and actions to kill and harm shall be forbidden for all eternity.
Use up all the Command Seals, and let the Servant finish himself.
Now I've only seen up to Episode 10 but from how this reads it seems to have been in place before even the first fight between Saber and Lancer since by episode 10
Sola-Ui Nuada-Re Sophia-Ri is Lancer's Master and probably doesn't care about Kayneth since she loves Lancer
However, Kiritsugu had tried to kill Kayneth on more than one occasion and there seems to be nothing that tries to stop him from breaking this Geis. in Fate/Stay Night in one of Heaven's Feel's bad endings, it's shown what happens when Shirou goes against the Geis he forms with Rin
Once he plans on going against Rin's wishes of killing Sakura Matou, he finds that he cannot move his body at all to stop her. He is immediately freed upon Sakura's death, as there is no longer any reason to bind him.
and in Cú Chulainn's legend, his demise was caused when he was put in a situation where he had to break a Geis
- Never refuse a meal offered by a woman
- Never eat dog meat
He was offered a meal of dog meat by a woman and as such he had to break one Geis.
So while I may be jumping the gun here and asking this before watching the entire series, where does this Geis with Kiritsugu and Kayneth appear? If it was before the first fight between Saber and Lancer, how come there was nothing enforcing the Geis as there was with Shirou?