How does the activity of Sn chemical potential vary as a function of composition at 900 C?
Just above the line, in the two-phase field where $\epsilon$ is in equilibrium with the liquid, the chemical potentials of both Cu and Sn are constant (very likely not equal to each other mind you).  This is a consequence of the tie-line on the Gibbs free energy diagram - check out a book on binary alloy thermodynamics such as Porter and Easterling, Phase Transformations in Metals and Alloys. 

At the peritectic point itself, the tie line connects all three phases, $\epsilon$, $\eta$ and the liquid, so the chemical potentials for each element are equal. Dropping the temperature just a smidge leads to two separate two-phase regions, one of $\epsilon$ with $\eta$, and one with $\eta$ and liquid.  The chemical potential of Sn (or Cu) is different in each of those two-phase regions.

Again, Porter and Easterling would be a good reference.