So the question is what should a nonagenarian, whose main goal is preservation of capital, be invested in?

In addition to TIPS, I'd look hard at 30-year bonds that were let 20-ish years ago when interest rates were above 5% and at preferred stocks with coupons (not yield) above 5%-6% and call dates at least 3 years away. The 10-ish years left to maturity of the bonds roughly match MIL's likely maximum life left, and an annual ladder of bonds maturing at one or two or three year intervals should be buildable. The preferreds will get called and need replacement, and that's some work, but the work tends to be bursty.

Also a heads up: MIL's IRA is no protection from being over-aggressive in her portfolio. The concern of a market deep downturn applies to IRAs as much as it does to taxable portfolios (and to Roth accounts, come to that), and at 90, your MIL has no time left to recover from a deep drop.

OT, but related: "recently widowed:" Has your MIL never been interested in her investments, or is she starting to check out?

Eric Hines