<human>: To start of my mom and my step-sister's (SS) relationship is cold at best. She stopped living in our house a few years ago because of all the fights they would get into. 

This afternoon my mom was driving through our neighborhood shopping center to pick up a birthday present for my step-dad, and she sees my SS's bf making out with another girl. She confirmed it was him by the jacket he was wearing, so I'm pretty sure there's no question about that. She also confronted him later and he said it was just a family friend. But the kind of kissing they were doing was not family-friendly (sorry, had to throw that pun in there).

The issue is: 1. My mom is afraid that if she tells my SS, that the SS will just use this as another reason to hate her.
2. Do you have a duty to tell someone they are being cheated on? I know I would like to know. 3. She can't tell my step-dad for at least two more days or it will ruin his super special birthday surprise.

Can you provide a brief overview of the text?
<bot>: My mom saw my step-sister's bf cheating on her, and doesn't know whether or not to tell her because of their already shaky relationship. Advice welcome!