Dreams about water parks often symbolize fun, playfulness, and the need for relaxation or escape from daily pressures. The endless slide might represent feelings of being trapped in a situation or a cycle that feels never-ending. It could reflect concerns about responsibilities or challenges that you feel you can't escape from.

Your younger brother in the dream may represent your protective instincts or a sense of responsibility towards family. Encouraging him to go on the slide could indicate your desire for him to face challenges or experiences, but the slide's infinite nature suggests uncertainty about the outcomes.

The encounter with the couple and the woman taking pictures could symbolize feelings of competitiveness or insecurity. Fighting with her might indicate frustration with others' perceptions or societal pressures regarding appearances. Your tired left arm could represent the emotional or physical toll of the conflict or the situation you find yourself in.

Overall, this dream may be prompting you to reflect on responsibilities, your relationships with family, and how external pressures affect you. Consider if there are areas in your life where you feel caught in a cycle or if you're feeling overwhelmed by expectations, and think about ways to address these feelings.