Not unusual, was your bro gc when you were younger too?
I've got the exact same thing - the patterns of childhood being repeated on the grandchildren. It's my parents are the ones missing out.
My sister is the gc, bro is invisible, I'm the sg. The grandchildren are treated by my parents according to what the parent was. Impossible to change people like this.
You can call her on it if you want, but chances are things may improve for a short while then slide back to the norm for your family (at best) or your child will be punished in some way for your dissension (at worst).
I'm vlc with parents and nc with sister altogether (in part due to how she treats the children).
"He is scapegoated for every misdemeanour that occurs while in the house with my nieces, even when they have been witnessed as instigators etc." I've had the EXACT same - dd has even been blamed for stuff that happened when she wasn't even present!! No! Not having that.
As a result dd hates aunt, is also nc with her (dd now adult her choice), doesn't much like grandparents either and rarely bothers with them. On the rare occasions she sees them she gets her ear bent how wonderful my sisters kids are! She'd maybe have more to do with bro & his kids if they lived nearer but they're a good distance away.
"Anyone who doesn't think my kids are equal to their cousins doesn't deserve to have my kids in their lives." Exactly.