My son who is 21 was diagnosed this week with type 1. He started to lose weight about 4 month ago. I think he got COVID-19 and I see the same with your brother. I hear about huge increase in Type 1 diabetes cases during the last 3-6 month and COVID 19 could be the cause.
Oh, I am so sorry to hear that, I know how you must feel and I'm sending you a big, virtual hug!
COVID-19 could definitely be the cause and there is even a new international registry for cases of newly diagnosed diabetes in those who tested positive for the virus (so called COVID-19-related diabetes) led by a group of researchers from the King's College London:
http://covidiab.e-dendrite.com
Here's also a short article on this topic if you're interested:
https://www.nejm.org/doi/full/10.1056/NEJMc2018688
They still don't know whether this type of diabetes will have the same characteristics of an actual type 1, type 2, etc., or some entirely new type of diabetes...