Polygamy occurs frequently. According to the Islam a man can have four women. Sometimes a man has more than four women, but that is not according to the regulations and is considered indecent. Sometimes the system of family relations is therefore complicated. When someone speaks of his brother, it might just as well be his half-brother, or his cousin. If you want to know precisely, ask after the relation to his brother. You will often hear that his brother has the same father, but not the same mother. In many cases it will be the same father and the same mother, because there are not that many Gambians who can afford to have several women.
There is often a second (younger) woman if the marriage doesn't bring children. You can easily inquire after the family relation. A Gambian will not be surprised. He will often voluntarily explain that it is his brother of another (or the same) mother. Don't be surprised to hear that they have different fathers and mothers. They grew up in the same compound, probably under the same roof where, at any time, their fathers decided to live together as brothers, and their mothers as sisters.