DP has custody of DSSs - they spend 5/14 nights with their mother. The comment about DSS2 still reacting to his parents' split - their DM walked out on DP & the boys 5 years ago. DSS2 has no recollection of his parents being together.
But he could still be reacting to it. If watching you with his sister is his first experience of a mother living with her child, he could be feeling very confused about why his mother left, what that says about her, what that says about him, etc. He's possibly feeling confused about why you have stayed with his sister when his mother didn't stay with him. And spending time between two households is an upheaval even if a child is used to it.
He will have noticed when she left if he was two, even if he can't remember it now. And he had a couple of years to adjust to it being him, his brother and his dad and then all of a sudden, new mum, new sister, new life, new set of rules, he's not the baby anymore, he's seeing how a mother and baby relationship works and maybe wondering why his mum didn't stay like you have, perhaps wondering if you will leave like she did when your daughter is a little older.
Their DM has only spoken to me by choice about 3 times in the 3 years DP & I have been together.
These three years have given the boys a lot of change. Your daughter is a year and a half old, which means you must have been pregnant just about nine months after meeting their dad.
How soon after you met their dad did they meet you? This little boy was four when you came into his life, and in a very short space of time he had a new step mum and a new sister living with him at a time he might just have been starting to realise that his family a bit different to some others with his Mum having left.
It's bound to be a big adjustment to his life, regardless of what he can remember from before.
DD is told off for pulling, pinching, etc when she does it and it's her fault. Neither DP or I will tell her off for pushing DSS2 away when he's in the wrong.
I think you might still need to tell them both. DSS, stop getting in her face, DD it's still not nice to pinch people. At the moment you are teaching her that it's okay to hurt people if they annoy you.
I encourage DP to spend time with DSSs while I take DD off somewhere. DP gives DSS2 jobs to do to help him look after DD when I'm out of the house. As soon as I'm home from work, I take over DD's care 100%.
This sounds like there is still a division or split between you all. DP and 'his' boys, your and 'your' daughter. Can you go off with the boys and leave DD with their dad sometimes? Does DP take all the children sometimes? Do you?
It does sound like he might feel a bit excluded when you come home. Do you take over care for all three of them 100% or just your DD? Does your DP do nothing for your DD if you are there?
Can you not all just pitch in a bit with each of the children and let the boys help you both?
Perhaps if you leave DD to your DP for a bit and ask DSS to help you lay the table or prepare the meal one night and then another night ask the boys to entertain her while you and DP cook, then have DP and the boys get the meal ready while you and DD do something the next? Mix it all up a bit, make sure everyone is included in everything?