It's okay for you to disagree with what I've said.

It's not okay for you to proclaim to "know" my diagnosis (which you got wrong by the way), nor to guess how it came about.

Pfrog missed nothing and still stands by what she's said ...

It's not the initial offense that's going to keep the OP tied down in guilt, shame and disgust for years to come. It's going to be the daily living of the lie (to keep it hidden, covered up, secret) day after day & year after year that's going to cause him the most harm over the long haul.

As painful as telling the secret is going to be, and regardless of the outcome, it is the only way the OP is ever going to be able to truly be free from the painful emotions he's experiencing because of it.

There's a saying in recovery that we are only as sick as our secrets ...

These kinds of secrets not only have a negative impact on the person who's keeping the secret, but also on the person the secret is being kept from!

The sister may never be able to figure it out completely, but her intuition will know that her brother and parents are keeping something from her and, therefore, it won't be quite as secret as they think it is anyhow!

Sincerely,
Pfrog!