Need Upline to get credited commission when an affiliate signs in and purchases

I need a way to allow the upline affiliates to get the commission credited to them when one of their 1st-tier or 2nd-tier downline affiliates sign in to the members’ area and make a purchase. I have hidden my shop page so that signed-in members only can see it and make purchases. As it stands now, when an affiliate signs into the members area and goes to the shop page and makes a purchase, that affiliate, the one making the purchase, is the one credited with the commission. I don’t want affiliates getting the commission on their own purchases. Is there any way to fix this?