Is the every weekend thing because you live a distance away from him and his school so can’t have him in the week?
If so, the child certainly should not have his time with his Dad reduced by half so 4 days instead of the current paltry 8 days a month! Cannot believe posters would suggest that tbh! Way to make the poor kid feel even more rejected and his Mum left doing even more of the grunt work while working too.
OP, I cannot believe you’re blaming your SS or his mum for not going on a plane holiday. If she wasn’t comfortable with her 7 year old going abroad without her, that’s her quite reasonable perogative. You could have just told his Mum you’re going and if she refused to let him go with you, you should have agreed alternative arrangements for SS for the weekend you were away.
It’s no surprise you have felt like this since having your own DC, natural protection instincts kick in, tiny baby and what seems like a big grown up child even though he was only 5. I can understand your wanting your DH with you in the NICU for 72 hours straight as well but if you’d had other DC (which he did). he’d have had to leave to do stuff for them at some point.
The fact this unjustifiable resentment has turned into you not being able to stand your SS 3 years later is totally on you though.
You’re making him your scapegoat and I agree that the poor little boy likely wants to sleep with his Dad as he doesn’t feel safe and secure in your home probably because he’s picking up on your hate.
Be honest with your DH about your feelings for him.
Hopefully he’ll do the right thing and tell you to leave, and make a better choice of stepmum for both of his and your DC in the future. He can have them both on the weekends. They may get along better when they’re not picking up on your jealousy and resentment anyway. Your DS will soon pick up on the way you feel about his brother too if he hasn’t already.
You might not be so judgemental about SS’s mum having weekends free when you’ve been single parenting and working all week yourself.