Nowhere in the op dies it say his brother 'doesn't understand sex', just that he mistook the ops sex noises for an asthma attack. This is a relatively easy mistake to make.

Why?

Not necessarily. Many people don't/can't leave home until they're in their 20s

Not every family lives like yours. My husband and I live with our 80 yr old aunt, father is 65 and we are in our 30s...its a big farm house and privacy is still hard to come by. Just saying, OP could be a legal adult living at home to help her parents with her brother.

If OP is living at hone to help out her parents and brother then I would be wrong. If however OP is living at hone, regardless of age, because they are not financially able to live out on their own, then it stands to reason that they are also not financially able to raise a child. mine is not the only opinion, and no one had to agree with it, but in my own opinion if you aren't able to deal with the consequences of an action you should not take part in that action. No protection is 100%.

I see your point but, by that logic, a married couple in their 30s who live together but who don't want, or aren't ready for, kids at the moment should never have sex. It's your opinion and everything bit a bit of a ridiculous expectation

that's kind of how it works, yeah.