Am I overreacting ?

I need advice,..I always feel like the bad person for saying no when it comes to my fiancé’s 5 and 6 year old kids. But we live together and I feel like they should listen to me just as much as they do to him. His 6 year old daughter listens to me with no problem. His son, hates me. Yells at me, screams at me, gives me dirty looks. If looks could kill me, his son would have done it a thousand times already. He’s so sensitive with his son. He doesn’t get in trouble and never listens (not even him at times) and he still gets what he wants. Every single time. His son got a new toy and it takes a cord to charge. I’m totally okay with that. But I’ve thrown 3 different charger cords away that was in their bedroom because we trusted them to leave them plugged in the wall for their toy. Well we agreed no more cords in their bedroom because they aren’t only still toddlers, they destroy everything of theirs, and they don’t need cords in their room. Well his son wanted his toy plugged in in his bedroom and of course, my fiancé gave him what he wanted. And it’s a little upsetting because he doesn’t ever give his daughter what she wants. No is no. And she gets punished. Not his son. And I feel like his son treats my the way he does because he won’t put his foot down and put a stop to it. Gets a little smack on the wrist. “Tell her sorry and that you won’t do it again”. Tells me sorry, but still does it and nothing ever gets done. It gets tiring after awhile and sometimes it makes me not want to be around because his son just doesn’t know how to act and sometimes I don’t know how to handle it. Especially when he screams bloody murder at me, kicks and smacks me. Do I have every right to be upset, or am I overreacting?