Since that is a PHEV much depends upon the ratio of ICE to total miles is. Presently I am at 4000 ICE miles to 12,000 total miles using Mobil 1 Extended Performance. At this rate it would be pretty foolish and wasteful for me to change the oil at 16K or even 20K. So how many ICE miles did your sister drive out of those 16K?
It may not be foolish at all depending on the condition of the oil from start/stop cycling of the ICE. See @K24Z7&M20AFXS post 21,986 above as an example.
In my use case an OCI of less than 20k is most likely a waste. The ICE runs under two circumstances. The first of these is where we take the vehicle on road trips and the ICE powers the car at highway speed for hundreds of miles. Obviously that is pretty easy on a synthetic oil. The second case occurs in the winter when the temperature falls below freezing and I need heat. There are days when I commute < 7 miles to work with the ICE running. This is somewhat harder duty, but it is limited to roughly 15 weeks per year and it's generally not that cold everyday. The car might be subjected to 50 such short trips per year, but the ICE runs the entire time to produce heat. It might shut off at a traffic light once fully warmed up but it rarely cycles on and off while the vehicle is in motion.
It was very traumaticYou serviced a running 18yo dodge 2.6??
Finally an aftermarket air induction that makes sense. Did the tone of the engine sound change after getting rid of the stock system? Any other observable changes such as mpg or power increase?