Mono and leukemia

My niece has been sick since May of last year.She is almost 3 years old She runs a high fever about 3 times a month. My sister will take her to the Dr. and they will tell her she has an ear infection give meds. She will get better til the meds are gone then she will get sick again. This has been going on now since May. Last month they put tubes in her ears and said that would fix her being sick. Since then she has ran a fever 3 times. She is constantly tired and has balance problems. My sister took her to the dr yesterday and they did blood work and her all her test came back that she has mono and the dr told her that her daughter could have had mono this whole time. My sister has been reading now and is worried that her daughters symptoms fit leukemia symptoms and it is really worrying her. Please if anyone has experienced this let us know