The answer is: they have no clue.
What is presented is that there will be a great tribulation, which involves four parts: Jehovah will lead Gog of Magog (Satan) by his jaws to result in the turning of "elements inside the UN" on religion and its destruction, the turning of their attention towards Jehovah's Witnesses, the only organized religion that will survive the assault, and Armageddon, brought on by the massive persecution against JWs described as 'touching the eyeball' of Jehovah himself, turning his wrath against the nations and bringing about Armageddon, which will kill everyone not actively associated with JWs as baptized members, and even some JWs who are not strong enough spiritually will also be destroyed. The destruction at Aramgeddon is permanent and annihilatory.
JWs teach that when a person dies, his sins are erased and he automatically qualifies for a resurrection, whether righteous or unrighteous, JW or not. Some will not be resurrected, including Adam and Eve, the religious leaders of Jesus' day, and Judas Iscariot.
What happens afterwards is the second resurrection and the work of restoring the earth to paradise conditions. How this restoration will be logistically accomplished is never said. The resurrection will be gradual, with the Hebrew patriarchs likely returning firstly, and those recently died early, and everyone else later (so most do not get a full 1000 years of life and teaching). JWs say there will be a gradual return to paradise and human everlasting life (not immortality) will finally occur after 1000 years. Then Satan will be released from his abyssing at Armageddon and mislead a great number of people who have lived for however many of years during the 1000 year reign and they will turn on righteousness and essentially a second Armageddon occurs and they are all destroyed and then faithful humans will live forever (still not immortal though).
That is briefly a description of JW eschatology, however warped it may appear.
Your grandmother would then have to convert her thinking and behavior to whatever JWs decide is righteous and proper after she is resurrected, and also have to pass the final test in order to live forever.
Not much of the basics of these teachings have changed in decades. They all differ substantially from what CT Russell taught. (edited to correct one noticed spelling mistake)