There isn’t a simple answer to any of this, but I will share a few thoughts. It looks to me that what maybe started out as kid curiosity developed into an abusive relationship as the years went on. Lots of kids experiment with their sexuality with siblings or friends when they are young. Exploration of each other is not abuse; it’s innocent curiosity. But by the age of 7, most kids have been taught about physical boundaries and know that they are being “naughty” if they engage in more experimentation. It quite naturally stops.
You didn’t say if there is an age difference between you and the steps. If they were the same age, we could see the initial incidents as experimentation among innocent peers. But if they were older and getting you to play spin the bottle, they knew better and were using you.
As for the step brother who continued the behavior despite your efforts to stop it: That is abuse! There was nothing loving about it. He was using you for his own pleasure. The fact that he also made sexual overtures to your friends suggests that he had (and maybe still has) serious mental health issues.
Please don’t beat yourself up about what happened. Your responses at the time are not unusual or abnormal. Your participation does not mean that you are at fault.
You were desperate for the love of your family. It’s not unusual at all for a child to be confused about the difference between real love and sexual manipulation. Kids who need love will often comply with sexual demands — and may be led to believe that having sex means they are loved. Bodies often respond even when the mind doesn’t want to, which can be terribly confusing. Kids who don’t feel safe enough to tell adults (or who are convinced the adults won’t help anyway) feel helpless and often don’t tell.
As for your parents: They managed their fears about what might be going by moving you to a room next door. They may have thought or hoped that would stop whatever was happening. From their point of view, they were protecting both of you. It wasn’t enough, but it was an effort.
It makes absolute sense to me that you are having difficulty with sex and sexuality so many years later. As you have found, self-medicating with alcohol may make the act of sex possible but it does nothing to deepen love.
You do need treatment in order to reclaim your sexual self. You do need treatment if you are to trust another person enough to enjoy true love and intimacy. Fortunately, there are alternatives to paid therapy. Here is an article from our archives that might be helpful. I also suggest that you join one of the forums here at PsychCentral.
I caution you about writing a letter to your father without the support of a knowledgeable therapist or counselor. It’s important to have clear goals and to be prepared for whatever reaction such a letter might cause. It’s possible he will respond with loving kindness and an apology. It’s also possible he will deny what happened or blame you or not respond at all. Any such negative response might be re-traumatizing.
Until you have more support, it might be more helpful for you to write the letter but not send it. Sometimes putting things down on paper helps people get something off their chest. It sometimes also helps the writer organize their thoughts and perhaps come to some new understandings.
This is a painful past. I doesn’t need to result in a painful future. With good support from a therapist, you can get through this and have the life and love you deserve.
I wish you well.