Because everyone needs to hold a banquet when they get married, and then relatives and friends will come to give them a bride price to congratulate them. If two sisters of a family get married in the same year, then her relatives and friends will come to her house to send her gifts twice a year. The gifts given by relatives and people with very good relationships are generally heavy, and twice a year is also a burden for them, so there is a saying that a family can only hold a banquet once a year at most.
They are cousins.