I think too that sometimes when relatives, even though well meaning, can be the worst support people kwim? often they aren't prepared to be a true support person and just want to be there for the novelty value or don't realise what they are in for. I am not saying this is what your sister is like, but from the sound of it she is a pushy type of person and I think if you let her in you might start butting heads and that is the last thing you want.

I don't think it is unreasonable at all to not want anyone there as it is a very special time between a couple and I have seen it when I have had my kids that when people have too many waiting at the hospital, that it can impede on the staff too. So I think that they really need to be respectful of your wishes on this one.