It depends on how hard he runs them.
Some people run full marathons regularly, sometimes two or three in a month. Sometimes, they run two marathons in the same weekend. When Bob Fletcher turned 50, he ran a marathon every week for 50 weeks in a row. (You can read about it in his book "Spaghetti Every Friday.")
However, people doing this know it is important not to race these events. They intend to participate in them using an effort that is equivalent to a training run. If they were to race a marathon every month, they would soon burn themselves out.
There is a rule of thumb in distance racing: "For every mile of the race, avoid racing for a week." Following this rule, someone racing marathons at his/her best speed would run about 2 marathons per year. Half-marathoners, such as your brother, would race no more than 4 marathons per year.
If it looks like he will not be racing between his October half marathon and next spring, I suspect he would be OK. He might even be able to race all three, or run all three hard. However, he should monitor himself for signs of overtraining syndrome. (He can do a web search for it.) If his pulse rate starts to increase, or he has other signs of overtraining, he can participate in the following half marathons, but he would slow down, and treat them as slower training runs.