Modeling Relationship

There are likely a number of ways to do what you are looking for. 
My first suggestion would be try and delete the relationship between the Channel and Sales table. Then build a relationship between the Store and Sales table. In theory when you select a Channel it would then filter the Store table which would then filter the Sales table to get your result.
If that does not work, you leave your existing relationships and then you can build the relationship between Store and Sales and leave it inactive. In any measures you create that require the Store->Sales relationship you would need to use the USERELATIONSHIP() function.
Another approach would be to use the TREATAS() function to create a virtual relationship between the Store and Sales tables.
Hope this gets you pointed in the right direction.