I really can see this from both sides, @SpanishFork.
Put yourself in your in laws' shoes for a moment. They don't have the kind of relationship with your daughter that began with an excited phone call and a picture of a scan. They didn't spend weeks wondering whether today was the day you were going to go into labour. They didn't go to the hospital or your house to meet her when she was a few days old. They weren't around when she said her first word or took her first step. When she came into their lives she was already a fully fledged person, who probably doesn't call them Nanna and Grandad. And, even if they don't realise it, they are probably holding back from getting too emotionally attached to her because at least subconsciously they know that if you and your husband split up, they will probably never see her again unless they're still alive when her sister gets married. So even if they are kind to your daughter, or think they are being kind, they just don't see themselves as her grandparents. Because they're not.
There are some things it is not reasonable to expect them to do, such as saving into an ISA for her. But I'm guessing that the ISA is not what is upsetting your daughter right now, and she probably doesn't even know about it. If you can afford to save for your children, perhaps you could save into an ISA yourself. This is one situation where I think it's OK not to treat your children equally. You could, if funds allow, put some extra money into your elder daughter's ISA to compensate for the fact that she won't be getting anything from her paternal family.
I think it is also OK for them to want to have some pictures with only their biological grandchildren in, if only for the fact that if you and your husband one day split up and they no longer see either you or your daughter, they're going to want to have some pictures of their grandchildren at all ages that don't also feature a child who is no longer part of the family.
However, I think it would probably make a big difference to your daughter if they stopped treating her differently in ways that she notices. So it would be nice if they chose to display some of the pictures that she is actually in. If the next time she goes to their house she sees a new picture on display which is one of the pictures she is in with the other children, rather than one of the pictures she took of the rest of the family, or a lovely picture of just her and her sister, she would probably feel more included.
I think it is also reasonable to point out to them that when they have family parties deliberately planned for days they know she won't be there, or when they give her Christmas presents which are clearly unequal compared to the ones the other children get, she notices. It wouldn't cost that much money for them to get her the same sort of Christmas present that the others get, and to get her something on her birthday.
But it will probably rub them up the wrong way if this comes from you. So you and your husband need to be on the same page about what is reasonable to ask them, and approach them together. Or even him on his own, if you trust him to get the message across more effectively that way.
Assuming they're not bad people, it would be helpful to explain to them that even though they think they're doing enough to include her, the difference in treatment is really noticeable. And it wouldn't be so bad if she had a loving family on her dad's side and this was just an added extra, but she doesn't. And so being treated as less important because she's not a blood relative is just rubbing salt in the wound for her, because it's showing her what she doesn't have and never will, because her paternal family aren't involved in her life.