Am I being precious or is my daughter being treated differently?

[whyIsARavenLikeAWritingDesk]

I'm prepared to be flamed for this completely and I get that there are more important things to worry about but I'm starting to upset myself over it.
My daughter is 20 months old and has a second cousin who is 3, we've recently moved to be near in laws because of ailing health.
Ever since DD was crawling she's been told off for doing things her cousin is allowed, called a devil because she's very adventurous (we don't let her get away with everything) shouted at for nothing.
Since moving down her aunty has been taking her cousin out every week and smugly telling DD that cousin is going out and she's not, I know she's too young to understand but she will get to the age where she will.
Yesterday MIL was repeatedly telling DD to we on the potty when we aren't really potty training properly just encouraging DDs curiosity, she obviously wouldn't go so was called stubborn and naughty.
MIL was trying to talk to DD and seemed to forget her name but tried to call her cousins name, then freely admitted she'd forgotten what she was called.
DP thinks I have an issue with his sister which I do but that's a different issue.
Please be aware this ISNT a MIL bashing thread because before we moved down I had no real issue with them.

Call them out on it every single time.

If SIL says to your DD that they are taking their child out, and your DD doesn't get to go, say "Are you trying to brag? Or make her jealous? Either way it's very childish."

If your MIL calls DD "devil", say "I would expect that by your age, you would know that it's rude to call people names. Actually, I'd expect a five year old to know that."

Alternatively, you could ask "why?" Type questions every time they do it, make them explain themselves.

"Why do you think it's ok to shout at a small child?"

"Why are you calling DD names?"

You need to nip this in the bud.

Her GM has called her a devil & forgotten her name-wtf?

Is this part of the "ailing health"?

If not, why do you see her?