you absolutely should not tell your father. this is none of your business, this is not your burden to carry, and your mother should have NEVER involved you. i'd like to slap her silly.

go to her. speak frankly about it. she cracked the door, so now you get to kick it open and demand the story. you might find once you hear it, your burden will lift. (your parents may have had issues at the time of which you, as a very young child, were not privy.)

or not. if it doesn't [lift your burden], ask her if she ever intends to tell your father. if her response is no, then point out to her that she has now made her own son the accomplice in her life-changing lie and tell her that if she doesn't come clean, you can't be responsible for what will happen next. (i urge vagueness.)

let me be clear - i don't think it's your place to issue ultimatums. to say something like "if you don't tell dad, i will." the problem is, it's also not your place to spend the rest of your life lying for your mother when this is something she should've either taken to her grave or come clean about 12 years ago.

what i DO think is your place is to make very clear to her what her demand that you keeping her secret will do to you. will you tell your dad? will you find it impossible to be involved with the family? will it fracture your relationship with your mother? who knows. but something will change, which is why it's appropriate for you to say that you can't be responsible for what happens next if she tries to make YOU responsible for HER lie, one that you should have NEVER been made aware of.

damn. i really want to slap your mother, no offense. it's one thing to make adult mistakes in an adult marriage and deal with them in shitty ways. it's another to involve your child in them. goddammit.