How do I tell the adoptive parents of my concerns about bias without offending them?

I am a first(birth)mom. I placed my son in an open adoption at birth almost 7 years ago. My son’s adoptive parents conceived a daughter just 3 months after my son was born(and placed with them). Though I know that my son is loved by them and their community, I can’t help but notice a bias towards his sister. Often the adoptive mom posts more pictures of her daughter(on Face book) than of our son. Since I have had the fortune to have visited with my son, I know personally how difficult it is to get good pictures of him(constant motion). I have noticed that his sister seems to get just a little more than him in many ways. Now, I could just chalk that up to there being more for girls to do at Disneyland than for boys. Or that, when I comes to fashion, my son is probably like me in that he will decide to wear the same things again and again, for comfort. While his sister wears different things as much as possible, so it just looks like she has ‘more’. I can’t shake the feeling that their is a bias toward my son’s sister because she was born to them. The problem is that I doubt that my son’s adoptive parents, or anyone they know, are really even aware of the bias they have towards her. She is an ‘easier’ child to care for, while my son is starting to show signs of ADD, which both myself and the birth father have struggled with, and my son’s adoptive parents are aware of this. How do I tell them of my concerns without offending them? I know they are doing their best, but I think you can’t improve or change a behavior that you’re not aware of. All I want is to make them aware of the obvious bias I see without offending them.