While his business style might be unethical, your strategy of keeping your customers and clients confidential is the right way to go. You have the right to protect your business ideas and customers, in order to reap benefits from the hard work you have invested. You can also prevent the strain in your relationship with your brother by having a conversation with him regarding this, but it has to be in an honest and polite manner.
Allah is the Provider of sustenance; people can devise all sorts of plans, but Allah is the Ultimate Planner.
All we need to do is place our trust in Him: “And whoever places his trust in Allah, He is sufficient for him.” (Surah At-Talaq: 3)”