Ilovemyboy - I think you've got an opportunity here with your sis. First, say "god, are you sure you want to be here when I might go into labour?" and then describe all sorts of horrific labour stories, MN has quite a few of these to choose from (not really reading for a first-timer, I must admit). This may put her off the idea of being around when that's happening.
As you can't know til it happens when dp's two weeks off are going to be, can you ask her to hold off on any plans for after the baby's born? Then simply don't answer the phone
I should add, my 'must have' newborn essential is a phone with Do Not Disturb. This means certain key individuals (in my case my mum) could phone when they wanted, but everyone else was directed straight to answermachine. The other key essential is a hard disk recorder, should be obtained a few months in advance so you can fill it up with the good daft telly of your choice - which fills the dual purpose of making you feel better when you are feeling like death and means you don't have to move off the sofa when you have finally got baby off to sleep and are too scared to so much as scratch your ear.
I know we have been quite negative about visitors on this thread, and MKG has - quite rightly - been trying to redress the balance. I've said this before but I'm sure my mum could have made a mint as a post-natal doula. She is brilliant (having had quite a lot of practice, to be fair!). She never tries to take over things with the baby, does all the cooking and cleaning and dealing with visitors whilst you lie limply on the sofa, and best of all she takes baby out for a gloriously long walk in the afternoon, leaving you to have a few blissful moments of kip without having to listen out for baby. She's also very pro-breast feeding but completely non-judgemental if you are struggling / choose not to.
She rocks! Visitors like her are more than welcome
At the very least, I hope you can rope your sis into taking the baby out in the afternoons. In fact, mention that now and she might run a mile, as I think I would if asked to take sole responsibility for a baby prior to ds' birth.