OH MY GODDDD!!!!!!!!!! I totally totally get how you feel!
My sister has recently told everyone that she is carrying a super healthy and 46 chromosomed baby...and all I can do is cry abaout it, and feel jealousy! Its horrible of me, I know, but there it is... out in the open.... making me struggle! I completely understand what you are saying and think you are completely justified in your feelings!
But remember you have ALL OF US HERE who care about your baby and only your baby. Your sil's baby is insignificant to all of us! Yehaa for the PF girls! We can stick together!
Cross those legs and beg baby mac to be late. That can be the difference between you both! You will your LO to stay in while she evicts hers earlier!
Also - think of it this way... the youngest baby is always the one that everyone is desperate to see! Yours will be the youngest family baby. By the time Baby Mac comes, her baby will be old news!
Much love xxxxxx