need advice

I have a 9 1/2 month old. I'm currently helping my sister and I babysit my nephew who is 2 1/2 years old. my nephew screams and wakes up my baby. I have tried talking to him and explain that he has to be quiet so that the baby can sleep. he's the only child and is jealous of my baby. question how can i make him understand to let the baby sleep? Or should I speak with my sister? I know it's not my place to discipline him.