Marriage between relatives
Does the Bible tell us (and, if so, where?) that it is wrong to marry a sibling and/or cousin? Obviously, it was permissible (and necessary) for Seth and his siblings to marry each other. Was it still OK by Abraham's time for him to marry his half-sister? What about Jacob marrying his first cousin(s)?
In the early part of world history, marriage between people who were related in some close way was inevitable. As time progressed and world population grew, God regulated marriage for his Old Testament people of Israel. In the Sinaitic Covenant, God forbade marriages between close relatives (Leviticus 18-20). That took place some 300 years after Abraham.
For people of God in New Testament times, people who are not bound by Old Testament civil laws, we recognize our obedience to the civil laws which are established by governments (Romans 13:1-7). That includes laws that regulate marriage between people who are related by birth.
Ancient Roman laws forbade certain relatives from marrying one another (1 Corinthians 5:1, for example). Similarly, governmental laws today place restrictions on those who want to marry. In our country, these laws vary from state to state.