I was in this situation - my DPs DD rejected me in his life and eventually gave him an ultimatum to either have contact with her or a relationship with me - unless he dumped me, she refused to speak to him. Similarly to your DPs DCs, she was exposed to her mums bitterness at the end of their marriage, in which I payed no part, but was subsequently blamed.
If your DP can't get past this then I can't see how you can move forward together. My DP refused to be manipulated by his DD - if he had gone along with it I would have lost respect for him and we would have split as a couple.
My boyfriend and I are both second timers. He has two teenage children who live with their mother and I have two young daughters who live with me.
They refuse to meet me, accepting their mothers story that I split up their parents marriage. This is false.
We have been together for nearly 3 years yet we are no nearer resolving this issue. He seems unable/reluctant to move them forward saying that they wont be forced and they are old enough (14 & 18) to make their won decisions.
This is making life so difficult. He is unhappy as they wont stay with us and therefore without their Dad and his relationship with my children cant develop because he feels guilty/miserable and means he has to continually choose between us.
Can anyone suggest any way forward or give me some hope.