My daughter was just diagnosed with a well defined tumor with its own blood supply in the muscle near her sternum. Is this likely to be malignant?

It could be, need bx. If it is a tumor in the muscle, it could be a muscle cell tumor, such as sarcoma. But it may be within the deep tissues of the skin, and perhaps it is a benign tumor, such as a lipoma. If it is a growth that was not there before, first it needs to be assessed if fluid filled: eg a pus-filled abscess, or a cyst. If not, it needs to be removed and sent for pathology: no guessing, no denying.
No, but... ... Unless the radiologist is certain that it's benign, and get two opinions, excision would be a great idea. Your daughter is lucky to have a mom who cares this much. Good luck to you both.