My brother was sleep talking about my cousin stealing his fish fingers
Dreams about family members, like your brother and cousin in this case, often represent aspects of yourself. Fish fingers in dreams could symbolize something that is easily attainable or satisfies a simple desire.
Your brother sleep talking about your cousin stealing his fish fingers might suggest that you feel someone close to you is taking away something that you believe rightfully belongs to you. Alternatively, it could also indicate a fear of someone close to you taking advantage of you in some way.
Overall, the dream could be telling you to pay attention to your relationships with those around you and to be vigilant against any potential deceptive behavior or betrayal.