Maybe the son could get jealous of someone else impregnating his mother and gets more dominant with her?
Here is how that scene would probably end up:
Daniel: But Mom, I wanted you to have MY baby. I wanted to be my little brother's dad MYSELF!
*He angrily stamps his foot.*
Tara: Now Sweetie, what did I teach you about waiting for your turn?
Daniel: To be patient and gracious, and not to whine.
Tara: So what will you be doing now?
Daniel: Stop whining, and waiting for my turn.
Tara: You are a good son, Sweetie. Now
a) apologize to Grandpa for all that whining and suck his cock.
b) maybe you ask XY whether you can knock her up?
So yeah, while the son being jealous of someone else impregnating his mother is imho the most fitting scenario for getting any jealousy into the main cast, I just can't really imagine him dominant. I also don't really see Tara getting dominated much. I guess if I were to try and write a scene where that happens, I'd pick Susi for doing the domination.