You could possibly setup a column in your date dimension table that would specify the previous year member that would be used for reference in your calculations. This would then we configured as a member property in the dimension that could be used in your calculation.

The other way that I could think of would be to configure a SCOPE statement in the cube that would evaluate the current week member you are looking at. Based on this it could determine the number of weeks that are included in that member's year (52 or 53) and then you could determine if you want to look at the equivalent member in the parallel period or the next member in the parallel period.