You were originally the mistress. You said you didn't know he had a wife? (Or is she his girlfriend?) Did you know at the time he had a son?
The wife is still coping with the facts that
- her husband cheated on her, lied to her, and risked exposing her to anything he picked up without bothering to tell her. (Let me be VERY clear, I'm not saying anything about you--I'm telling you rather what it is to be in her shoes, to have NO idea what you're dealing with or what you've been exposed to because you can't actually believe anything your spouse says)
- he's still seeing this woman. Again, if you didn't know he had a wife, none of that is your fault, but it's still painful to the cheated-on spouse.
Now, after only four months, you also want to build a relationship with her son.
No, on every level. Especially if she has any clue that your attitude is
what's the big fucking deal?
It's her child.
That's what the "big fucking deal" is. Her offspring. Her son. Another human being who's going to be affected by all this.
Kids are harmed by having a revolving cast of boyfriends/girlfriends in and out of their parents' and thus their own, lives.