Have posted this on your other thread.
You have been with this woman all of 12 weeks and you are staying over regularly already?
The middle child has issues with this and YOU are on here asking us how to deal with HIM?
Sorry, but #1, this is not your business, though I'm kind of pleased you are taking an interest of some kind.
#2, it's way too soon for the analysis of these DC, they have all clearly suffered, and all this will get back to the father, he is going to go spare about it and the DC are going to be slap bang in the middle of it all. AGAIN
The boy doesn't want to see his mother hurt again, yes he is being male and possessive about it, my DS (4.10) is the same, against his own dad, but that is for ME to resolve, by reassuring him that we are all going to be OK and that he will be fine. That takes time, and gentle negotiation.
His hormones are racing, and his world is shattering about his ears and Mum has a new bloke in her bed in a matter of months since meeting him.
IF you pair ARE love's young dream and are meant to be together my advice will be easily followed and will not impact on your future together in the slightest. it will however improve the lot of the most impressionable and clearly vulnerable people in all this, the DC.
You both need to back the hell up right now. Slow down, only see each other when the DC are not there. You need to arrange fun afternoons out together after a while of knowing each other.
Unless you are childhood friends, you literally could be anyone, she doesn't know you properly in 3m to move you in or allow you contact with her DC. Likewise you with her and your DC.
If you are meant to be together you will be. But the over nighters have to stop immediately until the DC have got used to you, and that you have proved that you are trustworthy.
I'm not suggesting you aren't, but in 12weeks, how could you be a fully known quantity. And how could she be to you?
You are both old enough to know better, you are both being selfish. You are also young enough to wait and take it slow and do it right.
Sorry to be so blunt, but anyone can see these DC are not being best served by this situation, and they are powerless to do anything about it. That, doing the right thing, is down to you and your GF.