What "I" would love to do in such a situation, is to have it worded so the niece thinks she's getting a rather large portion of the pie. Make it look like a pretty even split too, i.e. person A gets $120k, person B gets $120k, person C gets $120k, niece gets $120k, whatever is left goes to charity. But then the will goes into how each amount is reduced; only the niece has an unpaid loan of $2,500, which after interest, late fees, mega late fees, much higher interest rate after going into default, all compounded over X years, now has an amount owed of...$125k. The $120k amount that has been willed to the niece can be applied to the amount owed by the niece, and any remaining amount will either be paid to the niece (if there was any left over) or forgiven (if there was none left over...and there will of course be none left over). The idea is to make it look like this was the original wording in an old, and occasionally updated will. This way the niece will realize that if she had paid back the $2,500, she would have gotten a BIG payday.
But of course this will just cause the will to be contested, and the others in the will having to wait to get their rightful inheritance (which will end up being reduced by legal fees...and possibly by a buyout to get niece to sign-off and release the funds). So, absolutely don't do this. Heck, I'd be leery of even something like "and to niece I forgive the $2,500 loan I gave her plus all the late fees and interest it had accrued over all these years" because all she has to do is say "oh, I paid, here's proof, so now give me the $2,500 plus late fees and interest it accrued over all these years." I'd just bequeath something neutral, like the Mainstays Dinnerware set she enjoyed at Thanksgiving or the Timex Atomic wall clock she seemed to like so much whenever she visited.