silvercuckoo, a history degree is very different from a performing arts degree. The former is highly academic, the latter tends to be vocational.
If you intend to enter the performing arts, then you need to accept that you will probably not be able to make a living from that alone: that is a choice you need to go into with your eyes open and ime drama students generally do.
A history degree is a very different kettle of fish. It is aimed at teaching a skills set that is far wider than any specific career and students who choose it often do so in order to acquire skills that are sought after in business, politics, administration etc. rather than because they have dreams of becoming historians. (Not to mention that chances of making it as a professional historian are also rather better than the chances of making it as an actor.) It's about things like analysis, attention to detail, organising large quantities of written material. So not necessarily unrelated at all to the fields they end up working in.
And tbh maybe your SIL isn't the greatest example you could have chosen. By the sounds of it, my dd has already made more money out of acting after one year's foundation course than your SIL has 20 years after her drama degree- how do you know that she could have had a successful career in STEM either?
My relatives and friends with degrees in History, Archaeology, Ancient Greek, Latin etc are all supporting themselves perfectly well, thank you. One is head of archives at a national library, another is manager of collections at a different archive, some are high-ranking academics, several are managers of commercial firms, somebody else is in publishing and doing very nicely.