My narcissistic aunt asked me not to tell her daughter she hates or her sister that she'll be in town. How do I tell her my Mom did already?

I am scared I will be in trouble with this narcissistic woman because my mother already told her sister (not the daughter). My aunt hates her own daughter and in return the daughter is hateful back. The daughter was adopted at age 5 out of foster care and did not mold into the golden angel that her autistic brother did. My aunt already had an older daughter who is perfect. 

The daughter is slow at catching on in life. I believe she is learning disabled. My aunt says she is "just lazy". She tells lies a lot and is unmotivated and that is all. Also when she senses a threat in conflict she pulls away from the person. Thus I guess she has done this to her mother. I think that last part is understandable. She was born to an alcoholic mother and sexually abused in foster care. If she misbehaved she was disciplined way over the top including by my aunt and thus the lying habit started. She stole food at first but was 5 at the time and that I understand is very normal. 

My aunt will not except anything but perfection. Her older daughter who is biological shines her mother's glory because the daughter is gifted, graduated from Exeter, and went to Harvard Medical School. The son makes my aunt look good as she can show him off as her "special ed son" she saved. Meanwhile if he does something like says something inappropriate she yells at him way too much. Now he is just real quiet like a robot. Then there is the 2nd daughter. This is the one my aunt hates as she is not perfect.