In my opinion, if there is anything between husband and wife, it should be said in time. If you don't think it's appropriate for your husband to help your brother write his thesis, just tell him directly and don't hide it. I have this idea for the following reasons.
First of all, you can help for a while, but not for a lifetime. In fact, it can be said that my husband helped his brother write a paper, and the final result was to help his brother graduate smoothly. Even if he helps his brother graduate now, your husband can't always help him pass the exam when he encounters problems in the future. In the future, his brother needs to walk alone, but this may make him form the habit of relying on others. There is a saying that it is better to teach people to fish than to teach them to fish. Cheating itself is wrong and not a long-term solution.
Secondly, my brother may blame your husband in the future. Now that you have helped your brother finish his paper, he may thank each other. When his life is not smooth in the future, he may blame each other instead, complaining that his brother helped him cheat, let him graduate in a daze, find a job in a daze, and live in mediocrity. By then, your husband will not only be thankless, but also affect the relationship between their brothers.
Besides, helping others write papers is a mistake in principle and cannot be made. Although we are brothers, we should have principles in life. Of course, we can help him if it's just a little effort, but helping the other person write a paper means helping the other person cheat, deceive others and get away with it. This kind of mistake will be made. Once something goes wrong, many people will be involved. In other words, the incident itself is risky and harms others and yourself.
That's why my husband helped his brother write his paper. I think it is wrong to tell him.