Kittens sucking on their mother at 9 months old isn't normal in the wild. They should be stopped, even if it comforts them.
They shouldn't be relying on their mother to this time. This makes them dependent and cats are independent creatures. They should be seeking YOU for comfort, because you're a human.
Besides, if they keep nursing on her, she'll probably get scratched or bittin mistakenly, and such things cause infections. Try encouraging your kitten by providing more play and less time to go nurse on it's mother. Also consider seperating the certain kitten from it's mom MOST of the time, but not ALL the time, and make sure you don't seperate her from her siblings so she won't feel lonely. Do that for a month or so and if it continues add some more time for it.
The reason why you shouldn't seperate the kitten from it's mother all the time is because the mother may forget her and start trying to kick her out of the house. You want them to like each other but not to depend on each other as a mom and duaghter. Your kitten needs to feel secure on her own so she feels she doesn't need her mother, but still likes and respects her (she's becoming a cat in a few months, so it won't hurt to make her more independent).
If you notice your kitten needs comfort, try providing it yourself. Show her affection and love and attention. Providing treats when she does a certain good thing is a good way to encourage her to become more independent.
Thank you and please take my advice.