On my hubby's side (3 boys) there are 4 grandkids that live near by, and 3 that live out of state. Believe me there IS a pecking order. It doesn't go by age, it goes by paternity, and as it happens, the boy gc top the list above the girl gcs. The boy at the top of the attention chain has a sister who is at the bottom of the attention chain.
When the sister (at the bottom of the chain) was first born, and the MIL and I were out shopping, she was busy shopping for the gson, while I was making suggestions about stuff for the gdaughter.
Is is right? No. I too have the 'everyone should be equal' mentality. When they come to my house, I treat everyone the same. Like they're my kids. However, when the in-laws are around its a different story.
The fact is that several decisions and a lot of growing up needs to happen in all of the parents in the story. Your son is a half-sibling to the daughter. You need to have a good relationship with the daughter. One where you treat her like a daughter equal with your son. Your boyfriend needs to treat both his kids equal and arrange to spend time with his daughter like he does his son. He may have a stronger bond with your son because he's a boy, but that's not an excuse. His blood runs through both of them. The Grandparents need to treat both kids the same. As the kids get older, and notice the differences themselves (this is happening in our family), there will be some bitterness between them. Its better if that is stopped now.
Good luck, you are not alone in this frustration.
M.