Take your time, and don't form a connection with either woman. They might live with it if you treat both the same i.e. don't commit to either until you've had time to consult and think.
You can still be a good father without having a relationship with the mother.
Sarah is trying to use the children to coerce you to choose her. Consult a lawyer as soon as you can regarding your rights, especially the about the child who isn't yours biologically. She must be a desperate woman to try and separate you from a child who considers you her father, while your biological child gets to see you. That on it's own would be a reason to stay away from her.
If you're right and S does relapse, your children will need you. Just make sure you've established your rights to see both of them. It's worth fighting for.