Speaker A tells speaker B that he feels like he's seen her before, speaker B responds with:

あんたアレでしよ？哲也兄がこっちで舎弟にしたっていう

And speaker A replies:

哲也兄！ ？ っていうか俺そういう扱いなの！？舎弟って、俺あのハゲに兄貴分らしい事何一つされてないんですけど！？

What I'm confused about is why does speaker A call Tetsuya "哲也兄" - Tetsuya nii when the 兄 suffix is used for older brothers but then she says Tetsuya is her younger brother "舎弟", there's something I'm misunderstanding here but I can't pinpoint what. My best guess is that I'm probably misinterpreting 舎弟, which could have another meaning that's not in a dictionary and doesn't literally mean younger brother?

I would be grateful if someone could point out and explain where in going wrong, thank you!

Speaker A tells speaker B that he feels like he's seen her before, speaker B responds with:

あんたアレでしよ？哲也兄がこっちで舎弟にしたっていう

And speaker A replies:

哲也兄！ ？ っていうか俺そういう扱いなの！？舎弟って、俺あのハゲに兄貴分らしい事何一つされてないんですけど！？

What I'm confused about is why does speaker A call Tetsuya "哲也兄" - Tetsuya nii when the 兄 suffix is used for older brothers but then she says Tetsuya is her younger brother "舎弟", there's something I'm misunderstanding here but I can't pinpoint what. My best guess is that I'm probably misinterpreting 舎弟, which could have another meaning that's not in a dictionary and doesn't literally mean younger brother?

I would be grateful if someone could point out and explain where I'm going wrong, thank you!