Let's say you join the religion and your brother thinking that you know what you're doing, decides to get baptized as well. He then decides to do his own research and finds it is not the truth, discusses this with the elders and gets disfellowshipped because he will not change his view.
This would mean that you will be required to shun him, you will not be able to speak to him, you cannot socialize with him or have anything to do with him. He will no longer be a part of your life and it is you that needs to cut him off, you have no choice and you are not allowed to secrectly associate with him. If a serious family matter came up and he needed to be informed then you would be allowed to communicate this to him, but nothing else. How would you feel about this?
This is no hypothetical situation, it is happening all the time and a reason why many are here trying to warn others or the heartbreaking consequences.