If] two deaf-mute brothers were married to two deaf-mute sisters, or to two competent sisters, or to two sisters, one a deaf-mute and one competent; or, [if] two deaf-mute sisters were married to two competent brothers, or to two deaf-mute brothers, or to two brothers, one a deaf-mute and one competent —- these [widows] are exempt from chalitzah and from yibum. If they were not related, they should marry, and if they [then] wish to divorce, they may.