Bike Frame Size for MTB Leisure Rides
Strangely your wife's proportions are very close to mine, except I am a little taller. Speaking from experience, in terms of mainstream bicycle fit curves, this  a somewhat long inseam for height, which means 


You need a relatively taller stack
You need a relatively shorter reach (a relatively taller inseam means a shorter torso).


This is essentially the worst combination for fitting, which means ordering sight unseen will be difficult to guarantee the best fit. Being marketed with the words "race" makes me concern that the frames may not have enough stack, but looking at the geo chart suggests the stack is decent.

All that said, I think either the 17" or 18" could work, the 19" is too big.  Both the 17 and 18" have nearly identical stack heights, but differ in front-center dimensions (and reach) due to differences in seat tube angles (the top tube length is also nearly identical between the two). 

The two sizes also use different wheel sizes.  The 17" uses 29er wheels (larger), a slacker seat tube angle and therefore as has a shorter front to center measurement and a 1 cm shorter reach, while the 18" uses smaller wheels (27.5) and a longer front to center measurement. Perhaps this is to offset handling characteristics of the different wheel sizes?  I would almost phone to confirm the geometries.  

Choosing between the two may mean picking a wheel size.  The larger wheel size will be a bit more stable, which could be good for leisure riding. It also has a shorter reach which is good for a shorter torso. Therefore if I had to hazard a guess (danger) I would suggest the 17". Depending on how your legs are proportioned (i.e., femur to tibia) you may need to move the saddle forward if you have long tibias/short femurs, as a shallow seat tube angle is better suited to someone with longer femurs.

Given that you haven't tried the bike, also be prepared to have to change the stem.  Given the long legs, there is a good chance of long arms which could mean a longer stem. Also given that long inseams effectively shorten the stack height relative to someone with shorter inseams, you may need to use a riser stem and possibly a riser handle bars to get the position high enough. A quick check of arm length would be to measure your wing span (an outward facing wall corner can be good for this) and compare it to your height.  Most bikes are proportioned as if the two match.  If you have a proportionately longer wingspan you will likely need a longer stem so that you don't feel cramped, if you have a shorter wingspan you may need a shorter stem so you don't feel stretched out.  Finally, realize that when you flip a stem up (aka riser) you loose reach, so with more aggressive stem rise you may need to find extend the stem length as well. 

Geom Charts