Refer her to this 2007 org publication where the word afterlife is used in ¶ 2, an admittedly rare use in WTS pubs:
https://wol.jw.org/en/wol/d/r1/lp-e/102007442
It might be fun to watch her reaction, esp. if you portray it as her disagreeing with the org...
Maybe some day they'll change it, like they did with the word religion (1951, I think).
Grace is always undeserved kindness. They work very hard to separate themselves from the churches in general, including in vocabulary. But there are plenty of exceptions: Bible, holy scriptures, Hebrew Bible, progressive revelation, original sin, the inerrant Word of God. You might have some fun with your SIL and these topics.