I would like to respectfully suggest that this gun should be serviced by an expert. I say this for the following reasons:
- This is a quality firearm which is not currently working correctly.
- The firearm will be used for defensive purposes, so reliability is the most important factor.
- Revolver springs are not as simple as they seem.
Probably the most common cause of light strikes is someone "tuning" the double action trigger pull by lightening the springs. However, there are a variety of mechanical problems which can also cause light stikes. Most of the time these mechanical problems can be overcome by a heavy mainspring, but not always.
So if this revolver will be used for defensive purposes, I would recommend that all the mechanical aspects of the revolver including the springs should be examined and given a clean bill of health. Most of the time these checks will need to be done by a gunsmith, but there arew some experienced revolver owners who can do a proper mechanincal checkup at the "kitchen table." However, the people who can do this work will invariable have the following items on hand: an accurate trigger scale, gunsmith screwdrivers that fit the revolver, feeler gauges and knowledge how to use them, spare revolver springs. If no close friends have all of these things, then a gunsmith is the best choice.
If you are interested to know more about revolver springs and triggers, I have written a long article on the subject. This article has a subsection discussing many of the possible causes of light strikes.
The article is here:
Improving the trigger on Smith & Wesson Revolvers
And it is also discussed in this thread:
https://thefiringline.com/forums/showthread.php?t=584772
Good luck with your model 10. They are great guns, and when you get it properly sorted, it will serve you and your sister well.