My 54 year old sister has 2 thyroid nodules that are heterogeous and calcified. Is this cancer? How serious is this?

Biopsy . The nodules should be assessed by ultrasound and then biopsies done as indicated. Without biopsies everything is speculation.
Depends. She should have an ultrasound an needle biopsy. Calcium in nodules can be associated with cancer. Best to have it evaluated.