M2M Relationship tricky situation

[Lasselakan]

I have a problem that I've not seen any example of how to solve regarding PowerPivot and Pivot Tables. I have 3 tables; stores, visitors and store_competitors.

'stores' looks like this:
store_id, store_name

'visitors' looks like this:
visitor_id, visited_store_id

'store_competitors' looks like this:
store_id, competing_store_id

Now I want to be able to pick a 'stores'.store_name (e.g. in a slicer) and then illustrate how many visitors each of that store's competitors has had. The problem is that I cannot add Relationship between 'visitors' and 'store_competitors' on visited_store_id = competing_store_id since both columns can contain duplicates (a store can have many visitors and many competitors). I can however create a Relationship from each of these to 'stores'.store_id but that doesn't give proper result, it acts as if the Relationship doesn't "reach" all the way from 'visitors' to 'store_competitors' (i.e. if I slice on a specific 'store_competitors'.store_id then COUNT('visitors'.visitor_id) is not properly limited to the competitors of that store, but instead shows all stores in the whole table and shows counts as 0 for everyone and Excel also suggests that a relationship might be needed. I guess it's asking about a relationship between 'visitors' and 'store_competitors' but how can I add that if both columns have duplicates?)

Here is a more specific example with example data:

'stores'
10 storeX
11 storeY

'visitors'
20 15
21 15
22 16

'store_competitors'
10 15
11 15
11 16

If I pick storeY in the slicer I want to see the result as competing_store_id(15) had 2 visitors and competing_store_id(16) had 1 visitor.

Please give me a hand on this one. I'm familiar with SQL but find PowerPivot a bit tricky in some aspects like this one...