Probably the first one. You say you took on a parental role for your younger brother, but you say that is too broad. Be specific and share a story of something that happened between you that shows how you took on this role. Or maybe you remember a specific time you felt you had to be like a father to him in your example and how that affected you mentally/emotionally? Add some reflection about how that has made you a better person and what you have learned from it.
Just some thoughts, but I definitely agree that the cat one is not significant. The third one could be okay if you could share something specific, but I personally would go with the first. Good luck!