OK, it's obvious that she will reincarnate again, I thought she could be that elf, but she is too old and too OP, it would be impossible for jack to defeat her. But the little girl has the same age, and assuming she got reincarnated at the same time she died, she should be about 6 years old (back into the imouto form).
I must say that I thought it was strange how the first 3 chapters felt like a oneshot, I mean, they barely met and were already in a final fight with his sister losing hands, legs, and skin, I guess that was just the pilot and now the author wants to take things slower and properly (I mean there were so many plotholes in that 3 chapter that was barely believable, why did she come 20 years before when they died at the same time, how did she find him as soon as she got reincarnated, and what was her plan for hiding him, his family is rich, it would be so easy).