You have two choices, really. Either one is liable to piss her off, since she's being so pushy about it. One, you can either deliberately not call her when you go to the hospital and let events unfold as they may; or two, you can tell her flat out (once again) that you are not allowing ANYONE in the room except your husband and that you intend to have the hospital staff back you up. And believe me, THEY WILL. Labor and delivery staff does not screw around when it comes to the welfare of the mother and child, and they would physically bar your husband from the room if that was your wish - so she doesn't stand a chance.

Me personally, I would probably choose option one. I assume you will call your parents and probably your husband's parents, but beyond that I wouldn't bother calling a damn soul until that baby is born, all is well, and you feel like messing with it. You are going to be TIRED. Enlist the help of your mom NOW, before you deliver, to help keep sis out of the room so that you don't have to stress about it. Let them deal with her in the waiting room; you'll have enough to do. And FYI, just because she's had 3 doesn't mean she's an expert. DON'T make the mistake of letting her dupe you into believing that YOUR delivery experience is going to even vaguely resemble any of hers, or that your baby will have the same temperament, preferences or personality. YOU are the mom here, so go on and take charge.

Keep in mind that EVERYTHING that happens is at YOUR discretion and is YOUR perogative. If someone wants to get their panties in a wad about it, that's their damn problem. As my grandma used to say, she'll get glad in the same pants she got mad in.

Good luck, sweetie! I hope you have an easy time and a gorgeous baby!