Jamie Lynn Spears says she "didn't have the courage to speak out" when Britney was suffering from "debilitating exhaustion".
The former 'Zoe 101' actress admitted she was 11 when she first noticed signs her sister was struggling, and she realised something was different".
Appearing on 'Call Her Daddy with Alexandra Cooper', she said: "Britney was changing in front of my eyes.
"I was still too young to comprehend her struggles, but I was old enough to recognise destructive behaviors, similar to our father's.
"Something was different with my sister... I didn't have the courage to speak out."
Jamie claimed she became "an afterthought" in her family after getting pregnant at 16 and then being legally freed of her parents' control before giving birth to Maddie, now 13.
She explained: "I was so burned out on myself. I was not speaking to anyone. I was kind of just an afterthought, like they didn't tell me anything."
Meanwhile, Britney found out Jamie had given birth by reading it in a magazine, and the 30-year-old star was stunned her parents didn't tell her.
She added: "Britney was obviously going through her own s***, but I think I was so absorbed in my own s*** that I couldn't even think about that."
Jamie and Britney, 40, have recently exchanged insults over social media over the former's book 'Things I Should Have Said', but the younger sibling has since reached out to her older sister in an effort to end their dispute.
She wrote on Instagram Story: "Britney - just call me, I have attempted many times to speak to you directly and handle this privately like sisters should. But you still choose to do everything on a public platform. (sic)"
Jamie rubbished Britney's recent suggestions that she's been making false claims about her sister in a bid to sell her new book.
Her post continued: "In the meantime, please stop continuing the narrative that I haven’t been there for you or that I’m making things up. I’m happy to share how many times I’ve reached out to you, supported you and tried to help you. This is embarrassing and has to stop. Love you."