Welcome to DTP @jba
Im so very sorry to read about your brother. Personally, I would wait to tell your mum face to face. I would only tell her once.
Please keep posting as you’ll get lots of support here.
What a tough situation made harder by the difficult times we are currently living in. As others suggest, face to face will help you ‘share’ this sad news and gauge from her reaction how much she understands. When my sister died around 5 years ago, we told mum who was also around mid stage. She was sad but didn’t cry. We were concerned she didn’t understand but she did in her own way. She often tells the Carers at the home when they ask how many children she has, she always says 4 but she lost a daughter, so even at a now very advance stage, I know she feels the loss but the moment of sadness passes quickly which I have to say is a relief!
Be kind to yourself to as you have lost your brother .