Oh dear, let me tell you about weddings....
The first wedding anyone goes to is pretty great. Your own wedding is really exciting. Every other wedding under the sun is a PITA.
They are expensive (think travel, accommodation, present, outfit, day off work for those not on a weekend), take up half a weekend (or holiday allowance), and boring.
At a certain stage in life people can be lumbered with one practically every weekend which is unaffordable, and invasive. Sometimes it is nice to just get the housework done and relax at a weekend rather than jump into a car and rush off, again.
At a later stage in life they are a childcare nightmare.
When you have been married for years and experienced 'life' you can't help sitting there thinking 'those poor bastards.... they have NO idea'.
As for ones set in beautiful locations in the middle of nowhere - they are the worst. You have to drive, so can't even drink, or get a cab which will cost a fortune.
So stop being paranoid that your poor SIL will grab your limelight. She won't. the main thing that will distract people will be life - how they are going to get home/pay the hotel bill/get away from the boring f*rt they are sitting next to/avoid the member of the family they are having a feud with.
Finally, don't change your date to fit round her - like she needs the pressure of knowing she HAS to turn up, whatever, because you changed the date especially for her. If she has a new born she won't want to go. Think about childbirth - it's rough. She may be leaking from places you'd rather not think about, in pain, and unable to sit down for long periods unless it's on a rubber ring. Plus she may be breast feeding. Some people are happy whipping 'em out in a corner - I personally wouldn't be, and neither would I want to have to squeeze my post birth tummy and huge leaking breasts into something remotely wedding-y.
Relax, accept this is your big day, enjoy it, but be aware that for a lot of people kind enough to make the effort to show up it is just another obligation, and while they are delighted for you, they might well be more delighted for you if they were somewhere else.