J.F.
As the mother of a child with a severe peanut allergy (visit www.delaniesdoggy.org for more info on our story), I want to thank you for being so understanding. Many people falsely believe that their own kid can still have these things as long as the allergic kid doesn't eat it. Its not that simple, and you seem to understand that :)
Honestly, I wouldn't make a big deal out of it. If she likes an egg with cheese on it, go with that. You can try Nutella (providing neice isn't allergic to hazlenut as well) on her toast. Even try plain strawberry jam on her toast, my kids love that. Try Nutragrain bars instead of granola (did you know that even without the nuts in the granola bars, granola itself is in the peanut family?). Panckes, french toast, waffles, toaster strudel, pop tarts. I wouldn't even try to explain it to her (I also have a 2 1/2 y/o...my allergic one), as they often don't understand that they simply can not have something at this age. I assume you are staying with your sister, so if your daughter questions where her peanut butter is when you give her toast, simply tell her that there isn't any here, and move on.