I would just sift the top few inches to be sure there is no kitty poo in it.
What vegetables or other plants they used to grow there doesn't make much difference. What would make a difference, if you are concerned about that kind of thing, is if they used poisons (pesticides, herbicides, etc) on their plants. Do you know how long it has been since it was gardened? Your mom moved there two months ago, but presumably it was vacant/ unused for awhile before that. Many of the typical garden poisons have short-ish half lives, so will be considerably reduced if a year has gone by since anything was sprayed there.
Add a bunch of compost to it. Compost with live microbial/fungal/etc cultures helps remediate a lot of problems:
" Compost bioremediation has proven effective in degrading or altering many types of contaminants, such as chlorinated and nonchlorinated hydrocarbons, wood-preserving chemicals, solvents, heavy metals, pesticides, petroleum products, and explosives." https://www.epa.gov/sites/production/fi ... oremed.pdf
You will need to buy compost now, but start your own compost pile for later. Best thing you can do for your garden!
Check out the Compost Forum viewforum.php?f=35
for some threads on basics of composting and a thread on Aerated Compost Tea. Brewing up some compost tea would be a good way to add a bunch more beneficial microbial life to your soil.
Give it all a little time to work. That will give time for weed seeds to sprout, which you can turn under for extra biomass. Then plant!!
Best Wishes and keep us posted how it goes!