Is there any idiom that if you use it in your conversation, the audience would notice that you are accusing someone of having a double standard behavior or policy?
In Persian we use this idiom: "same roof, two (different) weathers".
There are two etymologies for this idiom, I'll give you the simple one:
In the old time when there was still no electricity, most people used to sleep on the roof of their houses in the hot summer nights.
There was a woman who had a son and a daughter. One night they went to the roof to sleep as usual. Each one slept in their own bed. In the midnight the woman woke up and took her daughter's sheet/ cover (?) and put it over her son, while was saying:
"Oh my sweetie, I'm worried for you, I hope you won't get cold."
Her daughter heard her words and said: "But mom, what about me? Why did you take my sheet? The mother replied: "Because it is hot, so you'd better not use that sheet in such hot weather."
Then her daughter said " Oh my God, but we are on the same roof, how is it possible that there is two weathers in the same place? (i.e. Mom! You are treating me and my brother unfairly while we are both in the same situation.)
Now, I will give you an example in which this idiom can be used:
A woman treats her own daughter and her daughter-in-law in a different unfair manner. Both of those girls are pregnant, but this mom recommend her own daughter not to lift heavy loads or she might have a miscarriage; however she asks her daughter-in-law to help them to lift a heavy load and says it's useful for women to have physical activity during their pregnancy. This mother has a "the same roof, two weathers" (i.e. a double standard) policy or behavior.
Is there any idiom that would mean someone is treating two other people/ group unfairly while they are both in the same situation (i.e. that person is applying a double standard to those two people)?