Boyfriends sister has a 105 fever and is vomiting. Could it be allergic reaction to bipolar medication?

Can be SERIOUS. I am not sure what medicine she is on. If she is on antipsychotics, it is important to consider neuroloc malingnant syndrome, which is a medical emergency. I would suggest to take her to er for evaluation unless there is other clear reason for fever.
Not likely. A fever that high with those symptoms can be an acute emergency. I have never seen a medication cause a fever like that. Get to a doctor or an er asap.
Not likely. A fever that high & vomiting in an otherwise normal person, presumably a teenager, is not associated with medication, but more likely with influenza or an acute illness. It's best that your boyfriend's sister seek medical attention.