I'm so annoyed

My boyfriends 15 year old sister just found out that she is pregnant. After his mom told me she said DO NOT TELL DAVID (which is my so) he pretty much raised these kids until my child was born. These are the kind of people that would start something and say oh well your girlfriend knew the whole time which would cause problems in my relationship. She then goes on to bad mouth me and make me seem like I am such a bad person. HER 15 YEAR OLD DAUGHTER IS PREGNANT BY A BOY THAT HAS BEEN LIVING WITH HER ON AND OFF SINCE SHE WAS 13. The boy is now 18. I'm just so beyond annoyed to the point where I don't want to even tell her when I go in labor or even have my baby number 2. I understand she is the grandma but I am so upset that she would even ask me to keep that a secret. Am I wrong for telling him?