Supervised visits only. There was a court case where some JW grandparents used their granddaughter's mom (who was never a JW) to be allowed unsupervised visits. The mom was fine with visits until they started the cult indoctrination, but they wouldn't stop so she only allowed supervised visits. Of course this went no where, the courts uphold parental rights in religious instruction unless there is something wrong with the parents.
You have your husband for an excuse so just make it clear she cannot do this and if she doesn't listen she will loose unsupervised contact. Children are impressionable, and this stuff is toxic.