#okay!!!

So... What this is referring to is the breaking news broadcast in reference to Jin, where they refer to him as 長男, AKA Eldest Son.

In English this carries an implication that there are Other Sons, or even Other Daughters, or so on so forth. When you hear someone referred to as Oldest (Something) you automatically assume that there are Younger Ones to compare it to, which is also what route the wiki takes.

However, this isn't the case for Japanese. While 長男 can imply Other Children, and often does in conversational Japanese, this term does not always mean younger brothers exist. What this refers to is a sense of succession, for one, and exists as a general term in order to refer to someone in paperwork, regardless of whether they're an only child or not. If some dude has an older sister and no younger siblings? He's the 長男. If someone doesn't have any siblings? Yep! 長男！It works for both. So can it be that Jin does have younger brothers? Yes, technically, but it's not explicitly implied with this line specifically.

How can I be sure of that, though? Wouldn't it help to have another example, you say? I fully agree! Which is why I have one! Here it is!

Here is Ritsu calling himself the 長男 of the Shinjo family! By this logic, we could technically assume that there are other Shinjo kids too, right? But then in Episode 16 he states Directly that he's an only child.

So in his case, he's both an only child and the eldest son. It sounds weird in English, but it still works naturally in Japanese. Another thing to note is that this line in Japanese has him refer to himself as 'The only Son', which Once Again could open the door for 'But wait!! What if he has siblings!!', but there's also nuance in That Word (一人息子) has less of an emphasis on the 'son' part and more of an emphasis on the 'only' part.

'Only son' in English has an unspoken emphasis on Son. If you hear someone say they're the Only Son of their family, and your first language is English, you'll probably think to yourself 'Oh, they have sisters then,' because that's what that phrase implies. The Japanese phrase, however, seems to emphasize the 'only' part. If you were to hear this phrase, you might think 'Oh, he has sisters,' but you could just as easily think, 'Oh, he's the only child'.

You can think of it as like.... Adding context into the noun. They are an only child, who happens to be a son. The only son. It's the same with 長男. They are the oldest son even if they are the Only Son.

So while it's not explicitly stated in canon that Jin does not have younger siblings (whether thats brothers or sisters), it's also not really truly implied he does. At least in Japanese. And there's really no good equivalent to the whole 'eldest son' thing in English anyway. Successor, I guess, is the closest, nuance wise? I guess? Shrugs.