Sorry to hear about what happened to you.
Unfortunately, at this time, the statute of limitations has run, and you cannot press charges at this time. In Oregon, your right to do so on an underage sex offense ends when the victim turns 24, and though you tried years ago, the state was not interested in going forward at that time. A prosecutor has the complete discretion as to what cases they take on and what they don't. And now, it's simply too old a matter for them to let you try again.
Similarly, it's way past the time that either the civil or criminal
court could go after your mother for her failure to protect you.
Without being able to secure a conviction, the law could not impose a registration requirement upon your mother's brother. What you may want to do anyway, though, particularly if it gets you some closure is to go and talk to the police and/or prosecutor about it. Even though they cannot go forward on your matter any more, if he's continued to victimize other children along those lines, you may be providing them with evidence they could use.
As far as your personal feelings about your mother are concerned, that's something that you will need to work through and for which a lawyer's services are going to be inadequate. You might want to post that part in our Mental Health category as a new question, where another type of professional may help you to understand what may have been going on in her head and how best to try to come to terms with it.