I'm so confused...why does this always happen to me???

So like the other day my mom decided to listen in on one of my sister's phone conversations. I was sitting next to her so I saw that she didn't like what she was hearing. And she proved it by giving me one of her famous "be good, not like your sister" speaches. So like later I asked my sister what the phone call was about, not telling her that my mom had heard, and she said that her friend had led this annoying boy on to think that they were going to have $3x. Her mom had read her ims about it though and thought she was really going to do it (while she was just going to leave him hanging) and she was grounded. So like I was going to tell my sister about it a day later when my mom told me not to tell her. Should I drop a hint to her telling her to be more careful with what she says on the phone, should I just tell her straight out, or should I just not tell her at all. (My mom had dropped a hint before to my sister saying that there was a rumor going around town about $3x.)