Have you read OP’s messages? She has said on more than one occasion that she does not expect them to give their step-granddaughter money. In actual fact, she was trying to take on the entire responsibility of providing equality in this situation, by matching their saved money with her own. But they won’t share the information, so she’s not sure what to put aside.
She also accepted that her DH was planning to leave his estate to his biological daughter.
Are you aware that the original post is all about her eldest being excluded from a family wedding and not about her expecting both daughters to inherit the same amount from their father?
When you say OP put her daughter in this situation, do you think that OP had a conversation with her “DH to be” and he confirmed ahead of them marrying and having a child together that he and his family would be spending lot of money on biological children, but not the daughter she already had? Do you think if she had had that conversation with DH, she would have married him and had more children? Again OP has said that he has treated both daughters the same in day-to-day life, so there is no reason to expect DH to suddenly say they will be treated differently, 5 years after having his biological daughter.
I do agree with you that DH should share the blame,
however. He is responsible for setting the tone with his family in all of this.