When I married my DH I already had a 4yo dd, I then had a ds with help and we went on to foster, years on we now have 2 more DC who are part of this family we also have a DGD who was one of our DC and was then adopted by our DD. There are 3 more DGC. All the DC who came through this house were accepted into the family, mostly staying for at least a year, some longer, everybody was treated the same by my DPs and PILs as well. We often as a family quip that there's no point looking for a blood match. We believe you don't need that to be a family to care for and about each other.
However our DD's birth father and his entire family which included a new step mum and new step siblings for her didn't feel the same and she spent years being left out despite our continual invitations to all family does that revolved around our DD and DGD's. It was all one sided. All those birthday, Christmas, school play etc invitations we sent. Mostly accepted and attended by her father and stepmother albeit in the latter's case with gritted teeth and our knowledge that afterwards would have lots of nasty backbiting going on which we ignored as best we could. Since her father died nearly 2 years ago nobody from that side has contacted our DD at all or returned her calls, her stepmother has ignored all the DD and DGDs birthdays, Easter and Christmas since and even tried to ban them from his funeral, having managed to exclude them from his last 3 months of life and the chapel of rest, she has never so much as offered our DD a keepsake of her DF. We can only imagine what happened to letters, cards and presents sent to the step mum. Its very sad but they seem to have come to terms with it now. We have never understood how people can be so difficult and put it down to unwarranted jealousy and fear. All you can do in your situation is offer support to your DGD, if you rush in to talk to her DF and trying to get him to 'have a word ' with his DW it won't go well. It will be taken as criticism and be used against not only you but your GD as well. Just encourage her to keep up pleasant contact with him and her half siblings making sure she gives lots of compliments to her step mum about the DCs. Even if it's not reciprocated. Then go off and do something nice for herself without dwelling on conversations or looking for double meanings in them. Her DF seems like my ex, lazy, happy to have a DD as long as someone else is making all the effort. In our family we have 2 mottos 'go with the flow' and 'if you can't say nothing nice don't say nothing at all'. There's really no point making things worse, looking for criticism be it intended or imagined or arguing, it just delays any chances of happy times.
But a word of advise, slightly off thread: make wills to ensure that everything goes where you want it, without wills blood suddenly becomes very important to the legal profession. My OH made his so that our DD and our DGDs are treated the same as our DS and our other DGCs if I die first. Without this will she would get nothing despite being our family for 35years.