They might have been working at night and taking classes during the day. I believe it was reported that they spoke decent English. Could they have been doing help desk/call center type work from their apartment at night while attending classes during the day? Then came Covid lockdowns and classes were probably moved to virtual as well. Which led to further isolation.
I‘m leaning towards the dominant sister having mental health problems which resulted in murder-suicide. Or, something along the lines of shared delusions leading to some kind of accidental suicide.
Seems to me that plain old depression could easily be a factor here.
Maybe the two young ladies were raised in luxury, with servants, and had insufficient life skills?
Knew they wanted out of the repressive system, but weren't prepared to really take care of themselves, really Adult in a new place with new systems?
And
then the global pandemic came to the party.
Recipe for disaster, seems to me, with or without a gas leak or fumes which would make everything worse.
Certainly possible that they recognized/thought they recognized someone from SA, adding to the downward spiral.
Is there a time limit on the relocation services, their visas, anything like that?
They needed a host family, imho, but we Websleuths Aunties & Uncles can't be everywhere, right?
Re: fumes. I was driving our car to the dealer for scheduled service once, and a line in the air conditioner cracked. By the time I reached the dealership about an hour's drive away, I had mild glycol poisoning (very mild.) I just knew the car smelled a little funny, and I had a headache. Service tech got the hood open on the car, saw the cracked line, and came into the waiting area to find me. He gave me 2 bottles of water, told me to finish both within the next hour, the dealership called my husband and gave me a letter for my doctor's office in case any symptoms persisted & I needed further treatment.
This breathing fumes for 45 minutes, the problem discovered by the service tech quickly. If the ladies breathed fumes for weeks, who knows?
jmho ymmv lrr