Let's say the timeline splits sometime 1977 or later, which is supported by the fact many key people on the Island at the time of the Incident, including Benjamin Linus, Eloise Hawking, and Charles Widmore, are off the Island in the ALT timeline. Eloise Hawking, then the leader of the Others, supports Jack's decision to detonate the bomb because she thinks it will save her unborn son from coming to the Island in the future to only get shot by herself. The detonation of the bomb causes a split in the timelines: think of it as a (Y) there's one timeline then it splits off into 2 branches. The island did not immediately sink in the ALT timeline, but the bomb certainly caused time to start moving in a different direction. However, in both timelines, Eloise would have had Daniel's journal concerning events that happen in the future of the original timeline because she received it before the timeline split. Eloise from the alternate timeline would have her son's journal from the original timeline, and she would also be aware of the attempt to change time in 1977. Could this be why she seems to be aware of the original timeline in her talk with Desmond in "Happily Ever After?"