I have the same problem. Unless you have the brothers marked as brothers before you put them as married to the sisters, the only way I have found is to delete the brothers, add them in again as not married to the sisters, then click add partner/ex, click connect to someone already on the tree, and enter in the appropriate sister. If anyone else has an answer that is a quicker way, please comment. Another thing: the tree won't let you have siblings without parents, so you might need to add some for the brothers.