No you cannot make a 2018 contribution before April 15, 2019 to your wife's IRA.
The IRS in a private letter ruling states that since the contribution is for retirement , the contribution is not allowed for the deceased.
In PLR 8439066, the IRS ruled that the estate could not contribute to the IRA of the deceased for the year of death, if the contribution is being made after the individual died. Had the amount be contributed before death, it would have been OK.
