D.M.
I don't have this problem with my own parents, because they're out of my life. But my in-laws? Oooooh boy.
My FIL is pretty good when it comes to treating the kids equally, though he does tend to favor my son. But when called on it, he's quick to fix it, even facing his fear of technology to play video games with my girls.
My MIL though? She's all about my oldest, while ignoring my other daughter and son. It would make sense if the other kiddos were the step children, but my oldest is the step child in the family, and they faun all over her like she's the second coming of Mary.
She loves starting conversations with, "So how's Eldest, her sister and that baby?" Or, "That boy." Which I've taken to, "Oh hubby's good, he just..." and then I rattle on about hubby's day. "No, I meant that boy." "My brother? Oh he's great! Finally got his eyes fixed and he.." "No! That BOY!" "Rufus? Well, he got fleas again, but we got that taken care of. Poor mutt's getting old though, so we have to be careful."
Until I finally, FINALLY get her to say my son's name.
There's no getting through to the woman. We bring it up, and suddenly everyone hates her, is against her, wants her to die! So, we've given up and just get creative. Fortunately grandpa has a clue and will make up with the kids, and try to fix the disparity.
The rest of the time? Even the kids look at me and say, "Yeah, grandma's nuts."