..I don't feel right about this, but here's you copy of the letter" on it so that she cannot reuse the letter in her efforts to find her husband a job by lying to other prospective employers in your name. Granted, it would be unfortunate to have to lie to your sister and to engage in subterfuge to keep peace in the family, but she is the cause of that...

Therefore, the best albeit imperfect solution to the problem described would be to avoid lying to the new employer at all costs.