It sounds like you all are mostly wanting to protect her assets from inflation rather than making major market gains. Her median life expectancy is 10 years, for reference. That means you should aim for a potential 15-20 year life horizon. An all fixed-income allocation may not be a good idea, so look into both an index equity and index bond fund.
For equity:
Schwab's total stock market index fund, SWTSX has a very low fee (0.03%). They have an international index fund (essentially Japan & Western Europe, so not as high-risk as developing market funds) with a reasonable fee of 0.06%. I'd probably just go with SWTSX to keep it simple.
Schwab has SWRSX, a low-fee fund that invests in US Treasury Inflation-Protected bonds (TIPS). The fee is half of the Vanguard funds suggested above. (0.05% vs 0.11%, or $50 vs $110 per year for $100k invested). Yields are about 3% per year, after taxes.
Their aggregate bond index fund invests in more junk bonds, and has marginally better performance in its short existence, so I'd steer away from that.
Between the annuity and social security, she may have enough fixed income that you can consider some balance between stocks and bond. For reference, I manage my parents' retirement accounts (about a decade younger) and have it 40/60 stock / fixed income. It's been stable over the last 15 years with 3-4% returns.
