Dylan O'Brien's relationship with his brother isn't as tight as he led some to believe during an interview last week ... 'cause his bro says they haven't spoken in months.

Here's the deal ... the actor recently did an interview with the Gay Times -- during which he said he was super grateful for his "trans, nonbinary sibling" before adding he knows other people who are queer and is grateful for how those relationships "deepen my experience in this world.”

Well, Dylan's brother -- who goes by Julz on Instagram and uses he/they pronouns -- fired off in the Instagram comments of the Times' post about the interview ... revealing he's "actually trans masc," not nonbinary.

He then added that Dylan hasn't actually spoken to him in over a year ... not even when he recently went through top surgery or for his birthday just last month.

Julz continues, sarcastically, "But glad I could deepen his experience in this world and make the world love him that much more