Just wanted to say, whilst I do feel sorry for the OP, it is not likely that a 17 year old would refuse to see her father on the urging of her mother. My experience of teenagers, and I have three of them, is that they will very much do their own thing and not be dictated to by anyone. For what it is worth, my father left my mother when I was 19. I did not meet the OW again (she had been my parent's next door neighbour and I had met her a couple of times before the affair came to light) and I severed all contact with my father from then on. I did this without any pressure being exerted by my mother. It was my own choice and I do not remember at any time in my life feeling so angry with a person as I was with my father then.
Feelings of adult children can be very complex in these situations. Yes, one loves both parents but also, when one parent does not want a divorce (and it seems clear that the OP's DP's ex did not, or why would she have used stalling tactics?) one sees one parent causing so much hurt and anguish to the other and it is extremely difficult to forgive someone for hurting someone one loves so dearly. If that makes any sense? There is an element of "if my father truly loved me he would not hurt someone who means so much to me" So, whatever people say about leaving the spouse not leaving the children, there is always an element of "he/she didn't care enough about my happiness" when a parent leaves. (Unless abuse is involved).
In my experience OW do not necessarily know what has gone on in their partner's previous family life, they only have their partner's word for it. Adult children, however, have a lot more inside information and will usually make up their own minds. The 10 year old boy may not be coming under pressure from the mother, or just the mother, he may be coming under pressure from his older sister too/instead. Not that I am implying that this is in any way right, just that you cannot assume that it is all the mother's fault necessarily.
I always think it is a bit unrealistic of second wives/partners to be upset when life doesn't run smoothly. I mean, you know the score and the potential baggage for the future when you embark on such a relationship. I understand that you would like everything to be picture book perfect but the likelihood was never very strong was it? We can't help who we fall in love with I suppose. I am not condoning ex wives who make life difficult for their DC and I'm not saying that I have no sympathy for new wives/partners but I can understand the anguish of an abandoned wife and can see how this anguish can easily turn to bitterness and I do sometimes want to shake second wives who seem to have unrealistic, not to say naive, expectations of how their future as a step mother will pan out.
There is only one thing that I found really irritating about the OP's posts and that is that I find the use of "DBD" particularly nauseating. The truth of the matter is that they are not your daughters and they never will be and they clearly don't view you as a "bonus" in their lives. I think you need to face up to that fact as unpleasant as it may be. You need to give your DP space and time to work on his relationship with his DS and you need be less demanding. TBH your DP's relationship with his son is none of your business. You need to back off on this one and enjoy the prospect of your own little one's arrival and the pleasure you will have in being a mother yourself. I hope everything goes well with you for the rest of the pregnancy and the birth. Congratulations, it's wonderful for you.