Leilani

I agree with @zil2.  The requirement for a legal adoption prior to performing the sealing ordinance is a modern Church administrative policy, but that doesn't nullify the ordinance itself. 
But there's something more important at play here.  God only honors the sealing ordinance (or any other priesthood ordinance) if, in addition to the proper formalities being observed by the proper authority, the Holy Spirit of Promise gives its ratifying seal of approval (D&C 132:7); which is conditional upon the parties' worthiness and their ongoing living in harmony with the covenants that pertain to the ordinance.  
Do you think the Holy Spirit ratified your sealing to your mother's husband?
I don't know anyone in your family, but based on what you say here . . . I'd be inclined to answer "no".  

I believe (but don't know) the sealing is valid regardless of the laws of men.  That said, a child molester will have a hard road to the celestial kingdom, and if he doesn't live up to covenants, and repent when he falls short, that sealing will be null through his disobedience.
More importantly, I'm so sorry for the pain this must be causing you.  I can't begin to understand, but I'm sorry and hope you're able to find peace somehow.
PS: Welcome to ThirdHour!

Leiliani,
The important thing is to remember that it is all in the Lord's hands.  His judgments are perfect.  We need to have faith and trust in him.  Once we refuse to allow Him the final judgment, we are really denying the Atonement.

My wife's parent's family is full of horrible evil stuff like intergenerational child abuse and incest. We're very happy to live a state away from all that drama.  I'm happy to say her wounds and scars have largely healed over time, and the stuff we hear coming out of that family doesn't bother her much any more.  She's still sealed to all of 'em (the ones that haven't gotten themselves excommunicated, that is).  I ask her about it from time to time, and her general opinion sounds like this: "They had their chance to have me in their lives on this earth, and in the next life it's God's problem."  She hopes they'll eventually be healthy enough to go to heaven, but she doesn't lose any sleep over it - she's busy worrying about herself and her family.
I'm pretty sure our doctrine states that heaven isn't a place where people can't stand to stand next to each other, much less be sealed to each other.  I don't know how such things will play out.  But I do know if I'm fortunate enough to get to heaven, and I saw her parents there, I'd be happy, because it meant they finally pulled their heads out of their collective butts enough to finally grasp and repent of their evil acts.