If they are all happy with the arrangement then it's OK. I suspect she has very little milk and it's more comfort suckling. I take it all are healthy and well and the kittens are growing?
Yes, very normal. .Mum will stop it when she's done.
Hopefully all are spayed/neutered by now. Male kittens can mate with their mother from 4 months, and females are best spayed before their first heat, again from 4 months.
Omg! I never knew they can mate with their mum from 4 mnths old! I left with 2 kitties female now. The male was given away but planning to take it back. I pitt my kitties going for spaying since they are still young. Thank you for telling me anyway. Thanks again everyone