I agree with others who suggest your sister take the buggy with her for getting around the airport etc. Essential! The ones for hire in the parks are basically like plastic bucket-shaped seats on wheels and not comfortable I'd imagine. Also, they're for hire just inside the theme parks so you'd have to carry the baby (possibly sleeping) to and from the car park or bus stop.
As regards baby food on the plane, Boots sell baby food in cans as opposed to glass jars and that's what I brought - they were great, no risk of breakage (they have the pull off ring-top thing).
You can get the spongy ear plugs in chemists - they don't stay in a baby's ears for long but they helped a lot on the plane for us and our baby (even though we had to put them in about 30 times

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Bring a lollipop or seven - the sucking action will help the baby's ears.
We brought 2 cans of formula with us as I did a bit of research and they don't sell the brand we use in the U.S. (Farleys). We used lunch bags and measured out the right amount of formula into them and mixed it with the bottled water when it was time - instead of carrying around the bottles made up.
Nappies (diapers) are basically the same over there (Huggies and Pampers) except cheaper of course!!
I'm not sure what the temperatures will be like when you're there but if it's hot or there's a strong sun I recommend a fan that attaches to the buggy. We found it invaluable. This is the type of thing I mean -
http://www.kidsstuff.com/g184.html
Be sure to have a sunshade too, as others have said.
Have plenty of drinks in the parks for baby - If you run out, they sell apple juice and water in almost all counter-service restaurants there and at some of those carts along the way.
I don't know if you'll have a rental car. If you don't (or even if you don't feel like battling traffic) I can recommend a great grocery delivery service which we used last year. They're called wegoshop and the website is
www.wegoshop.com - you phone or email them with a list of what you want and when you want them and they'll bring them to you with a very reasonable charge added for their trouble. We saved a lot of time and money that way by having water and juice delivered (as well as other things). Our girl loved the U.S. baby food - things like Gerber animal rusks!
There's a fabulous baby centre at each WDW theme park. These places are spotless and have huge changing tables as well as microwaves for heating food and first aid kits etc. Or just to go in and sit there for a while in the air-conditioned rooms is a treat.
Make the most of the baby swap system which is available at many attractions (basically those ones which have height restrictions) at WDW and at Universal too.
Have a great trip!