Is the wife sinful if her husband's brothers enter upon her suddenly while she is not wearing a hijab?

None of the husband's relatives are permitted to enter upon the wife, and it is forbidden for each of them to look at the other's face, even if temptation is assured not to occur. The husband's brothers should not enter his house except with permission and by knocking on the door. If an unintentional glance occurs, there is no sin in it.