Is it bad that my baby still doesnt sleep through the night?

my 6 month old still won't sleep through the night. my mom says this is a bad thing, and when my sisters baby was 4 months old she slept all night. i would say probably 3 out of 5 nights she wakes up. sometimes all it takes is for me to hold her and she is fine, but really, is it bad that she isnt sleeping through the night, every night? what age did your baby start sleeping through the night? how can i help gracie to stay sleeping?