dynamic relation between tables based on slicer

You can create inactive relationship and based os slicer slection, calculate your measure using any of the relation with USERELATIONSHIP funtion of DAX.

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River 

You can only have one static relationship between 2 tables, slicer can only work on report page. There is no way to dynamically change relationship in power bi. Other than the userelationship(), you may also use Treatas() and Intersect() function for virtual relationship.

USERELATIONSHIP function (DAX) - DAX | Microsoft Docs

Physical and Virtual Relationships in DAX - SQLBI

Paul Zheng _ Community Support Team