Verified answer

It depends. How much Sn is dissolved in Cu? If it is only a few ppm, the mp will not change enough to be measurable.

The mp or melting range of an alloy can be higher or lower than the mp of either pure metal. In the case of Cu-Sn alloys (and without looking at the phase diagram), I believe pure Cu is the highest temperature solid on the diagram.

The why or why not depends on the thermodynamics of the two elements involved.