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]])
Paul is speaking to the Corinthians as if they were one person, so all instances of "you" and "your" here are singular. (See: [[rc://en/ta/man/translate/figs-you]])
Paul uses a question to cause women to think deeply about what he is saying. AT: "you cannot know if you will save your unbelieving husband." (See: [[rc://en/ta/man/translate/figs-rquestion]])
Paul uses a question to cause men to think deeply about what he is saying. AT: "you cannot know if you will save your unbelieving wife." (See: [[rc://en/ta/man/translate/figs-rquestion]])