In the US, the first $5,000,000 from an estate is tax-free, including gifts given over the lifetime of the recipient beyond around $14,000 per recipient per year (that number changes every year). Assuming your uncle didn't have millions of dollars, then, you likely did not owe estate tax.
However, since this was from a retirement account, the money was likely taxed due to the retirement account being tax-free until distribution. This assumes this was a 'regular' retirement account (a Traditional IRA, a Traditional 401(k) or 403(b), or similar, which is funded with pre-tax funds).
If you and your sister were the designated beneficiaries of the account, you would have had the option to 'stretch' the IRA's funds over your lifetime, essentially giving you a proportion of the funds each year for your life expectancy. If not, or if you chose not to do this and instead take a full payout from the account, you would indeed owe income taxes on that amount; and since you were taking $240,000 out all at once (presumably $120,000 each?) you would owe quite a significant amount. I suspect $80,000 includes some other estate fees, but it's entirely reasonable for it to be in that ballpark, especially given you are a non-US resident.
My suspicion is that your uncle did not designate a beneficiary, and thus the account was liquidated as part of the estate. That's unfortunate - always designate a beneficiary for your IRA, even if you're only 25 and your IRA is low on funds, because it's easy to forget later, and the options for taking money out of the IRA are much wider (and financially better) when you're a beneficiary.