You can achieve that with Access Rules. But first simplify your domain model
if wenslijst_tussentabel, leverancier_product_tussentabel and bestelling_heeft_producten have no attributes they can be replaced with a reference set association
Add an Bestelregel entity with at least amount and unit
After that add an access rule to bestelling
for the Role Supplier
[Bestelregel/Bestelregel_Product/Product/Product_Leverancier/Leverancier/Leverancier_Account='[%CurrentUser%]']
This means that only accounts related to levancier can see the lines of the order which they supply.
Make a rule for a client/klant that they can only see their orders/bestellingen.
My answer is quite short, please read the documentation about access rules.