There are 2 girls who had a fight and are mad at each other.
One of them asks the protagonist if he is gonna fight against the other girl and he anwswers:
「あいつが邪魔をするなら戦う。
……まあ、ホントは聖杯なんて物騒なもんはあいつに任せたいんだけどな。
あいつならキチンと鞘に収められるし、桜を助けてくれるだろうし」
The the girl:
「……そう、でしょうか。
あの人は、魔術師です。
わたしみたいに弱い人間のコトなんて、考えてくれないと思います」
Looking online I found this:
元の鞘に納まる:一度別れた男女が、いろいろあった末に結局はもとの仲に戻る(復縁する)ことを言います。
I understand that it means something like get together, but it talks about man and women.
So, could this expression mean something like:
Make up, Make peace with each other
Even if it has no sexual meaning?
Alternatively it's talking about obtaining the 聖杯.
Another definition is:
鞘に収めた:強い働きかけを止める様子 Stopping a strong influence
I am not sure since 働きかけ seems to have multiple meanings. Looking it up I found this one:
人に対してその物事について考えるよう促すこと
Urging people to think about the circumstances
(Might be wrong how I understood it)
Or does it mean another thing completely?