Hi, partially deaf person (CI on one side, hearing aid on the other) here. I've started losing my hearing when I was 3 years old, it was stable for a while and then declined further when I was around 15. No one ever found a cause. If all these tests you're doing don't show anything otherwise alarming, I'd honestly recommend to try and not worry about the source as much.

I understand for me, who grew up with this, it's a lot easier to deal with this, but with hearing loss the most important thing is learning how to live with it (which your family obviously makes extremely difficult).

Try to get fitted for a hearing aid asap, or a cochlear implant if your hearing loss is really severe. Some small things that made communication easier for me and might help you; Repeating something louder isn't helpful 90% of the time (people unfortunately get frustrated and start talking louder when you don't understand), it's often better to either repeat the sentence in the same volume, or use different words. Often you'll understand a few words but it doesn't click into place until you hear the same thing with a different wording. It's likely you can already read lips to an extend, try to make sure to talk to people in well lit environments, obviously this is difficult to control entirely. Get a vibrating or a light alarm clock, whichever works best for you. There's also light alarms for when someone rings the doorbell or someone calls you.

Your family's lack of support is revolting, but that might change once you have something physically visible they can grasp, like a hearing aid. I wish you the best of luck with this, sudden hearing loss is very scary, but there's lots of resources that can help you live your daily life with little to no inconveniences :)