"You shall not cook a kid" — but the Torah speaks of cooking a kid specifically in its mother's milk. What about cooking it in the milk of an animal that is not its mother — say, an older sister?
The Mekhilta derives the answer through an a fortiori argument. The mother enters the tithing shed separately from the kid — they are counted as separate animals for purposes of the animal tithe. Despite this separation, cooking the kid in its mother's milk is forbidden. Now consider the kid's older sister, who enters the tithing shed together with the kid — they are counted as part of the same batch. If cooking in the mother's milk (separate tithing) is forbidden, how much more so should cooking in the sister's milk (joint tithing) be forbidden!
The reasoning is elegant: the mother and kid have a weaker tithing connection (separate), yet the prohibition applies. The sister and kid have a stronger tithing connection (together), so the prohibition must apply even more forcefully.
This extension moves the meat-and-milk prohibition beyond its literal wording. The Torah says "its mother's milk," but the Mekhilta shows that the prohibition cannot be limited to the mother alone. The principle extends to any female of the same species whose relationship to the kid is closer than or equal to the mother's. The "mother's milk" language establishes the paradigmatic case, not the exclusive one.