First of all, ditto what Blondie said. A person in a vulnerable situation has no defenses against what a cult can level against them.
I attended pioneer school back in 1986. We spent a great deal of time practicing how to talk to different kinds of people. For example from our "secret" pioneer book, "Shining as Illuminators in the World", p. 54 under the heading Choosing Bible Topics for 'All Sorts of People', it says: What Bible subjects do you find appeal to: men, women, youths, older people, religious people, agnostics, atheists. Then there are numerous sample discussions for each topic.
Added to this are tracts, brochures, magazines, and whole books devoted appealing to a person's particular vulnerability. One that stands out for me was the brochure, "When Someone You Love Dies". I always carried a copy in case I came across a person suffering the loss of a loved one. It very powerfully gave them the (false) hope of seeing their dead loved one back here on earth. It was difficult to resist.
I am sorry for you recovering that your niece became another victim of the cult. I myself was recruited as a teenager and spent most of my adult life in the WT clutches.
Steve Hassan says that everyone in a cult has an authentic personality and a cult personality. Try to reach your niece's authentic personality. Although I suspect as a newly baptized Witness she has buried it deep.
Please accept my sympathies.
Reopened Mind