Cherry80, you are just going to have to be patient I'm afraid. The DSS seems as though he still thinks well of you and your DH, so through maintaining a relationship with him, in time the girls may come round.
TBH, it sounds like the gap between the marriage ending and yours beginning may have been a little tight. Girls are funny about Dad leaving. Now there is another little one on the way. Ok so the Ex has been bending her DC ear, nothing you can do about that realistically other than to convey to the DSS and the DSDs that you are there, eager to meet them and welcome them to your home and your heart whenever they are ready.
Your comment " i dont want him spending every Sunday with his son without me once our baby arrives, i'll need more support off him." bothered me though.
You can not monopolise your DP's time. He will be with you all week, but one day, he needs to be with his own son. That boy comes before you. Given the rest of the story, he really needs to make sure that this remains the case.
If your DP were happy to dump all his DC and be with you, I'd be advising you to dump him, It seems however that he is a caring soul, who loves his DC, loves you and will love his baby with you. You need to relax, fit in where and when you can when it comes to his DC. For now. Play the long game. It'll get better in the end.
If you get all pouty and needy, that will only make things worse for everyone. Be smart, you have plenty of time for life to be as you would like it to be.