Very likely you can't do it, at least not legally...
A few things you might consider:
If your wife has no earned income and you file taxes individually, then she can use her personal deduction to offset the capitol gains tax. Doing this would mean she'd have to sell chunks of it over several years.
If the mutual fund was sold at a rate that was lower than the original purchase price, then you will not have any capitol gains tax. This seems unlikely if it was left to her by your grandmother "some years ago".
If you somehow have some major deductions to declare on your taxes. One way I can see this happening is for you to use the money from the sale to fully fund both of your IRAs ($11k), your 401(k)/403(b) ($18k each, or up to $36k) and HSA ($6,650). This could qualify you for the Saver's Credit. All this is doing is off-setting the capitol gains tax you'll pay on the sale of the $50k, and if maxing out your contributions lowers your AGI to under $74k (joint filers) you should only have to pay 5% LTCG - since its mostly bonds she probably won't have very much to pay in the first place.
Someone correct me if I've made an error here.
The first two seem unlikely unless you meet these criteria. The last one might be your best bet.
g'luck.