I've been maintaining a private database of RE shipment figures going back to 2005.
This graph compares RE2, RE7 (+GE) and the original releases of RE5 and RE6 in their first five or so years of sales (RE7 and RE:2 aren't there yet). The rate in which RE:2 is selling this early in its lifecycle is unprecedented for the series. It's pretty much inevitable that RE:2 overtakes RE5, even when combining all of its various re-releases.
Here's another graph of the same games, but with all versions accounted for (last gen remasters, Gold Editions, etc.). It's not a perfect graph because we're not sure how RE5 Gold Edition sold before Capcom abruptly reported that 2.3 million copies were sold by Q2 2017. But still, I can't see RE:2 not taking the crown in a few years.