Op, it sounds like, while you saw them as replacement grandparents, they just saw themselves being nice to their daughter-in-law's child.
She certainly deserves their kindness and respect, so if they aren't giving it to her, I think you should speak to them and make the point that you, and dd, appreciate the care they have shown her for the past 14 years, that she cares about them and it hurts her feelings when they do x,y,z. Obviously the cake incident is unacceptable - could it possibly have been a mistake? Did she know that she was definitely going to be in etc? If your MIL carry's on doing things like that on purpose, even after you have voiced your concerns, I would cut her out.
I don't think it would be appropriate to expect your PIL to feel the same love for both children though, and I also don't think you should demand that they provide financially for your dd in the same way they do their own grandchild. That's asking too much. They shouldn't be saddled with the responsibilities your dd's paternal family failed to meet.
Does your dd know that they are not her actual grandparents, or have you been passing them off that way all these years? If you have been honest with her, and they behave kindly and respectfully in their interactions with her from now on, she is unlikely to feel rejected by the savings issue. I don't expect anything at all from my step-mother, for instance. She has her own children. When she does give my dc's gifts or whatever I am genuinely grateful. I certainly wouldn't feel put out if she gave her own children something she didn't give me.
If you haven't been honest about who these people are to your dd, she is probably going to feel very hurt when she realizes. You will have to deal with that, but if that is the case, I don't think you can direct the blame for it towards your PIL.