Here "brother" and "sister" refers to a Christian husband or wife. Here "not bound to their vows" is a metaphor that mean the person is not obligated to do what they vowed to do. This can be stated in active form. AT: "In such cases, God does not require the believing spouse to continue to obey the marriage vow" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-activepassive]])