Why are sisters jealous and insecure of their brother's wife but the same sisters are not jealous of their sister's husband?

One way of looking at things:Typically it is an older brother that is jealous. It is usually because that is his little sis and he was always the one to protect her. So if the husband does anything to her it usually the brother who will support her. He is basically giving up his role of protecting her all the time. But do note that it is really not jealousy its more just a feeling love and protection. Another viewpoint:It is not true or correct to say that most brothers are jealous of their sister's husband. Some are, some are not, and no one has counted to see which group is the most. Making that assumption is uncalled for.