@Vortax
I feel you, you can't convince them to use Signal and even harder to use different OS which is for non techie same as Windows, they only use it for web browsing. What is your brother doing mostly on his computer? If its web browsing then Mint or Ubuntu are great for him. You can try to sell it as "non-malware" OS, and faster experience overall. If he is gaming then there is nothing you can do to persuade him, you will make him even hate Linux.