How do I determine the amount of Calcium in a substance using EDTA titration?
Yes, EDTA will chelate to both calcium and magnesium. However, since the complex formation constant for $\ce{Ca^2+}$ is significantly greater than that for $\ce{Mg^2+}$ ($\log \beta_{f,\,\ce{Ca^2+}} = 10.65$; $\log \beta_{f,\,\ce{Mg^2+}} = 8.79$), we are justified in assuming that EDTA preferentially chelates to $\ce{Ca^2+}$ and only chelates to $\ce{Mg^2+}$ when no $\ce{Ca^2+}$ is available.



Remark. How significant is "significant"? In principle we could avoid the approximation and solve for the system analytically, but then we would still have error from (i) imprecision in visual determination of endpoint, (ii) imprecision in determination of formation constants, and so on.