We don't know how old the cousin is, but if it's a child who knows he was adopted, then if/when he asks about his "birth tartan", the family might explain it by saying he can wear either (or both) tartans, emphasizing that the family chose him to become a part of them. Or, as Brook said, just don't make an issue of it, and when he's old enough to find out on his own, if he cares, then that will be his decision.