[X] "This reeks of a Caster Servant" [X] Something else, annoying as this is you better give it some thought so as to avoid even more questions. Thinking on it, siblings don't explain your interactions well at all. A half-sister, with Saber as the legitimate child and you as the bastard daughter would fit, but you have no desire to play the bastard. "Cousins, your aunt eloped with a bard of common birth. That should explain why I'm a 'peasant' while you are obviously of noble birth." Annoying, but less annoying than other options, and it would explain both the hostility and the trust you've shown in each other. [X] Let Saber decide on the plan, that's her job after all. You'll just hit stuff, and maybe burn it when she's not looking.
[X] "Are you honestly telling me that you'd willingly halve the number of insults you can fling at me, or did you not realize that you can't keep calling me 'peasant' and 'farmgirl' if we're siblings? No, we'll be cousins."