I didn't have my mum at my DS's birth. It was just me and DH. It was never an issue with our respective families - there was no pressure or hinting etc. I am close to my mum and she is medically trained but I just really wanted it to be an experience for DP and I for similar reasons to Briggsy's Girl. I don't think we're that uncommon

I think it's really important that you are BOTH comfortable and relaxed. You won't labour as well if you feel even mildly anxious about something or if you're picking up on your DH's discomfort. Sometimes its good to have a third person available as Jasp said but sometimes you really need to just be quiet and in yourself and concentrating and not get too distracted by other people around you if that makes sense. You won't know how you'll labour until you labour but you sound like you've talked about it enough to have a fair idea.

If you're both definitely decided that it's just going to be the two of you, it might be best to just be upfront with her now, acknowledge she might be disappointed but explain your reasons and leave it at that. You are not your sister - just because your sister had her there, doesn't mean she should have assumed that you would do the same (if in fact that's what's she's done). There are so many other ways for grandparents/parents to get involved.

ETA: Good luck darl!