What is "舎弟" supposed to mean in this context?

Question

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!

Answer 1

This sentence has some yakuza(Japanese Mafia) terms.

Yakuza treat their member as a family, so they use family words like 弟、兄, etc.

舎弟 is like a young brother for superior yakuza, 舎弟 call his superior yakuza　兄貴 or ～兄.

Answer 2

You seem to have missed several points.

• Despite what jisho.org says, AFAIK 舎弟 is never used to refer to one's blood-related younger brother in modern Japanese. Here 舎弟 means someone's subordinate/disciple/pupil/follower/etc. It's mainly regarded as gangster jargon today, although it may be more like a casual 弟分 in this context.
• You seem to have failed to recognize the ～にする construction. (～を)舎弟にする means "to take on someone as a disciple".
• Speaker A said "哲也兄!?", but that does not mean he calls Tetsuya that way. Speaker A just repeated in a surprised tone what speaker B said.

My translation attempt (although not literal):

B: 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
B: So, you're the person Tetsuya-nii took on as a disciple here.
B: Oh, I think I know you. Tetsuya-nii said he took on you as a disciple in this place.

A: 哲也兄!?
A: (You said) Tetsuya-nii!? (i.e., Wow, is Tetsuya your older brother?)

A: っていうか俺そういう扱いなの!?
A: Wait, is that the way I'm supposed to be treated!? (i.e., Do you mean someone said I am a disciple of Tetsuya?)

A: 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!?
A: What the heck is "disciple"? That bald bastard did nothing for me like a leader!

Answer 3

To interpret the OP's question, I perfectly agree with Yuuichi Tam's understanding of the background of the given phrases.

Yuuichi Tam said:

This sentence has some yakuza (Japanese Mafia) terms. Yakuza treat their member as a family, so they use family words like 弟、兄, etc. 舎弟 is like a young brother for superior yakuza, 舎弟 call his superior yakuza　兄貴 or ～兄.

In my understanding, 舎弟 is a younger brother-in-yakuza's-law just like to say a younger brother-in-law who is not related by blood. Same as 舎弟, 兄 or 兄貴 is an elder brother-in-yakuza's-law in the given phrases.

In my answer I am going to call 哲也兄 "the boss Tetsuya" instead of "the elder brother-in-yakuza's-law Tetsuya", and am going call 舎弟 "an underling" instead of "a younger brother-in-yakuza's law".

I'm going to tell another reason why 兄 in 哲也兄 does not imply a real brother of the speaker B later on.

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

This phrase is an inversion of

あんた（は）、哲也兄がこっちで舎弟にしたっていうアレでしょ？

こっちで implies こっちで勝手に, and こっちで literally means here and acutually it means from 哲也兄's side, and 勝手に means (to do something) without one's permission.
アレ is literally that, and in this phrase, it implies the man on the topic.
As a whole, the given sentence could be interpreted like:
Are you the man whom 哲也兄 boss Tetsuya made his 舎弟 underling by himself without your permission?

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！？

is roughly devided into two phrases:

1. 哲也兄！ ？ っていうか俺そういう扱いなの！？舎弟って、
2. 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！？

哲也兄！ What!? Is Tetsuya my boss!?
っていうか is と言｛い｝うか if I say in other words. そういう points to 舎弟って（＝ 舎弟という） in this phrase.

The phrase 1 is interpreted like:

哲也「兄」とは何ですか！　哲也から俺を見ると、俺は哲也の舎弟という扱いなの！

What!? Is Tetsuya my boss!? He takes me for his underling!

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！？

ハゲ is a word to be used to insult a person who is bald. In this phrase Tetsuya seems bald. The whole meaning of 2 is like:
I haven't been given anything that seems a favor from a boss from the bald bastard, (so I wouldn't like to consider Tetsuya is my boss).

The final answer:

I think Tetsuya is not the speaker B's or her older brother but is also her boss or something like that, because the speaker A called Tetsuya ハゲ in her presence. You can't insult a yakuza in the presence of his real sister even if the speaker A does not accept himself as an underling of Tetsuya.