she is not my mom.....

Have a sit down with your dad, and i bet both of you are about done with this piece of work.
Then have a sit down with all of you and spell it out for both of them.
She's the guest turned invader, not you or your dad. (the fifth wheel)
Lay it on the line, it's either her or you.
It's too bad it's come to this, but it sounds like it's going to totally go out of control if something doesn't change and NOW.
Don't let her come between you and your dad or it will snowball and get worse.
Find out what's she's on and find a way to get her out.
I know your dad is trying to help someone, but ask him what's the risk? Emotionally and psycologically.
Kick her the hell out, she's poison and you both know it.
Too bad, it could have been a good thing for both of you.
Just my 2 cents.
Good luck to you both.
Tom.