n400 - ~80 years old parents about ~50 years old children

[java.io]

I have a colleague applying for citizenship on behalf of her ~80 years old parents who live with her.
My colleague is USA citizen and her parents of course are residents for enough time to quality.
She needs help on the question about children, her parents have 2 children, her and one more child (her sister) who lives in Europe and is about 50 years old. She provided all the info in the application but she can not get the birth certificate of her sister.

Is the missing birth certificate gonna be a reason for RFE? Should she submit N400 without it? Or it's better to put efforts and try to get it!
Thank you!