Is marrying your cousin (if they are a believer) forbidden?

Answer From The Annual Reading Schedule - Week 27 Notes For Leviticus: If they are a believer – No. While many relationships are forbidden in Leviticus 18, a cousin is not one of them. We can see an example of cousins marrying in Joshua 15:16-17 where Kalev's (Caleb's) nephew Othni'el (Othniel) married Kalev's daughter Akhsah (Achsah). This is recounted again in Judges 1:12-13.