I wouldn't borrow it expecially since it is a new stroller. It's just going to get a little beat up at Disney with traveling and everything, and if it is a new stroller, that's not really fair to your SIL. It's one thing if it's yours and your family beats it up, but I know I'd be upset if someone brought back my new stroller and it was scratched and dirty. Not saying you will do this, but it could happen. Maybe I'm just extra picky about our things (I wash down our stroller and things all the time to keep them new looking), so maybe it wouldn't bother your SIL at all. I just know I wouldn't be thrilled about the idea. Plus I'm sure she just bought it because she needs it. I'm not sure how long you are going, but if you are going for a week, that is a long time for your SIL to go without her stroller, expecially if she has younger children.
My suggestion would be to look at craigslist or rummage sales for a used one. Then when you are done with it you can just resell it. Good luck and hope you have a great trip.
