My prescription for contacts is -4.25. Is is safe to get contacts that are -4.50!

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if it's just for a short period it won't be a problem, but over time your eyesight could get worse because of the additional strain caused by that 0.25 difference.

-0.25 is not that much so you won't feel anything more than a slight blur or headache.

You should also check the base curve of your sister's contacts to see if it matches with yours because the lens' curve is made in a way to fit properly on the cornea. You're going to have lots of problems if they are not the same BC and it might cause problems like corneal swelling or ulcer.

The slight difference in the prescription wouldn't be that big of a deal, what is more important is, do you have the same diameter and base curve? It should be marked on the boxes. If they are the same, and you want to try it, open some fresh ones, don't use ones that have been in her eyes.