She will be okay. Ds1 was 25 months when ds2 was born. Ds1 weaned during the pregnancy. I was fully prepared and expecting him to want to nurse again after the baby came. He did not show any signs of wanting to nurse until his brother was about 15 months old. I think that is when it got confusing to him. He had understood that babies need momma's milk, but not that someone who could eat solid food and walk and talk would need it too.
Sorry for the ramble, K is a smart girl she will understand and be okay ( and so will you).