I agree that there's pressure for dating JW couples to jump into marriage quickly before they get sexually involved. I'm wondering though if there might be some financial pressure as well - pooling resources, only having to pay for one residence, etc. depending on how your niece fared in the divorce.
Then again, some people can't stand being alone - they are terrified of being alone and will jump at the first chance to get back into a relationship. There's lots of pressure in JW-land, and I think some JW women prefer to have the decision-making responsibility in someone else's hands, and enjoy that role of Wifely Submission™.