I will admit to being stumped.

Starting female PCs have Distaff 10. (BoK&L pg 51)
BoM(pg 10) gives a Steward the starting skill of Stewardship at 5+2d6
KAP5.0 has...
- Damosel = Stewardship 15
- Lady = Stewardship 12
- Maid in Waiting = Stewardship 7


So what is the correct starting value for an NPC wife?

Whatever is the appropriate starting Stewardship for her culture. As she's an unnamed NPC, her skills won't increase until after she becomes "Sir So-And-So's wife".

Thank you, dude)) (https://rubrides.com/blog/dating-help/impressing-russian-girl.html))

The correct answer of course is:
Whatever the GM says it is. :)

I think the starting values are the base for the starting PK lady and do not reflect what a new Wife might have, necessarily. Good starting point to be sure but there is definitely going to be differences between 30 year old widow who has looked after her first husband's manor for 15 years and a 15 year old girl more geared towards a lady in waiting career due to a poor dowry despite her considerable personal charm.

In Book of the Entourage, 1d6+(Age-10) is used, up to the maximum of 15. This gives a nice spread of the usual first marriage wives (ages 15-21), who are aimed to marry vassal knights by their parents, and hence it stands to reason that they would get some training on manorial management.