Lover or Brother

I am 20 years old and in a relationship with a guy aged 24 years. My boyfriend is a brother to my cousin's husband so I consider him my in-law. My mother found out about the relationship and is not happy. I am not sure if we are committing incest or if we are going against taboo. We are not from the same clan and we are not blood relatives but everyone is against this. I love him and he loves me too but I need clarity on this.

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