Purpose of に in お 兄さん に 聞いて ください

Purpose of に in
お 兄さん に 聞いて ください

I think it shows who you’re asking, this sentence probably means “please ask (your) brother”. “聞く” can get confusing because it can mean “listen” and “ask”. For example: “お兄さんを聞いてください” would mean “please listen to (your) brother”. Hope that helps!