What is the danger of salmonella in 'home laid eggs' and how should I clean them?
The danger of bacterial contamination is much lower in home laid eggs than in commercially produced eggs exactly because of the washing process that eggs go though in the US.

See the accepted answer to this question:
How long can I keep eggs in the refrigerator?

Eggs are naturally laid with a protective coating on them that will keep out bacteria. An egg keeps for a long time in a nest after all.  

Commercially produced eggs are washed- I assume for cosmetic reasons- which destroys this coating and makes the porous egg shells very susceptible to bacteria.

You should discard eggs that have damaged shells but other than that you can consider your mother-in-laws eggs much safer than any you could get at the supermarket.