Thats more or less what I thought had happened...

I am sure others will agree with me that this is illegal and unethical (from a duty of care, loco parentis point of view because step dad or not he is still a legal guardian). Several things to think about:

1) If you (at some point) outed yourself (when you felt up to it, bear in mind nothing in BDSM is illegal) theh he would lose all this power over you.

2) Given the very suspect legality of what he is doing, you should be aware that he actually has more to lose being outed by you than you have being outed by him. Look at the recent case in Austria with the man forcing his family to live as prisoners - who was it who was splattered all over the tabloids as an 'evil man'? Also, how long do you think he would last with your mother (assuming he is still with your mother) when it is revealed what he has been doing to you, her daughter? Snowballs in hell is probably an accurate description.

3) BDSM is all about trust and consent. Blackmail is a common fantasy but it is never 'for real', the submissive consents to give the blackmailing material to the Dom/me. If you do not trust the person and consent is truly forced then it is not BDSm but abuse. If you were under 18 when this started then it may even count as child abuse.

Now, it is not my place to tell you what you should do in this case nor when to do it. However, I hope that the above makes you think and gives you ammunition if and when it comes down to it.