Normally yes. But in this context it's understandable. Is it the right thing to do? Absolutely not. But their situation is also far from being ideal.
He is a gay teenage boy who lost his entire family at a very young age and had no guidance or support at all to help him deal with his brother being kicked out, his parents dying, his brother leaving him again, then transitioning, then trowing a fit and refusing to explain why he needs to stay away from someone he is starting to have feelings for. Is a lot to process.
I guess he just needed to put all of that out. Plus technically his boyfriend clearly is involved in this mess, otherwise the sister wouldn't tell him to stay away.
Sorry, but I feel that secrets make you appear ashamed. When my friends tell me they are gay, it is just part of life. Same as any other choice. Getting married, having sex, getting divorced, cutting your hair, invetro, adultery, adoption, dieting. I talk to everyone equally and don't feel compelled to keep anything a secret, unless told to do so. It is life. Live it how you want and know that people will have opinions. It's okay.

Tbh i dont really like this chapter. Its really not his place to share his sisters story to whoever he wants, but thats just my opinion