"This is going to kill her," the friend tells PEOPLE. "In the end, she'll be supportive, but this is a big shock."
Another source says there's no rift between them, but that Jamie Lynn's advisors have tried to keep her apart from her 26-year-old sister.
"Britney doesn't have to listen to anyone now but her sister still does so who knows what people are telling her," says the source. "She has her career now too."