My sister is pregnant and she is only 15 she only told em and she told me not to tell my parents.....?

i think i should cuz the baby is coming in 4 months and she will need to tell them, me and her are really close so what if i tell my parents and she gets mad at me....and i dont wnat my parents to think that this was coming cuz she is really pretty but she didnt want to have some guy forced her, but i dont want to tell but i should, what should i do??????