Hello and thank you for entrusting me to assist you. My name is ***** ***** I will do everything I can to answer your question.
While I completely appreciate your concern and frustration, there is actually no law requiring employers to be fair or to pay employees with more difficult or time consuming job duties more. The only restriction is that an employer cannot pay one employee more than another if their motivation is a legally protected trait. Legally protected traits are things like race, religion, age or gender. So, if you could prove that you were getting the more difficult on call assignment BECAUSE of your race, or BECAUSE of your religion (which doesn't appear to be the case based on what you've described) it would become actionable under civil rights laws.
But in all other circumstances, your employer is free to be unfair, and to pay two positions equl even though one is harder. Your only recourse in this circumstance is to reason and persuade.
As for the operating room manager who has a sister in the same department, there is nothing illegal about that either. This is an unfortunate matter of office politics for which there is no recourse under the law.
I hope that you find this information helpful and am genuinely sorry if it is not what you were hoping to hear. Please do not hesitate to let me know if you have any questions or concerns regarding the above and I will be more than happy to assist you further.
If you do not require any further assistance, please be so kind as to provide a positive rating of my service so that I may receive credit for assisting you. Very best wishes moving forward.