No you can't sign on your wife's behalf. I would be very suspicious of a broker (was it an independent broker or a rep from a life company?) telling you to do this. As well as being unethical it could invalidate your insurance if the company compares her handwriting to a difference sample. The company would also argue that if she wasn't around then how could she have answered all medical questions correctly?
I would look into this and get proper signatures in place to ensure that you are covered. Having just gone through this process in the past few days, we both had to be there, we both had to sign once and we both had to answer medial questions for ourselves.