My mother died last week during heart surgery and the cause of death was an allergic reaction to protamine sulfate and she had allergies to sulfa drugs. Am I wrong making inquiries into her death?

I'm sorry for your loss.
Sulfa drugs (based on the moiety sulfonamide) and sulfates are not chemically related. So a person with a sulfa allergy does not necessarily have an allergy to sulfates.
Unless there are other indications that the doctors should have tested for protamine sulfate allergy, I guess there is no case. But you miht want to talk to an allergist.