the majority of it I'm not going to be able to do anything about it, but the present thing I can
Honestly, @Differentstarts I’m afraid I think it’s your responsibility to begin to manage the situation - because you already know you won’t be able to disguise it for much longer.
I'm not with the youngest daughter’s dad. We wasn't together long and his family barely know my eldest and don't have any kind of relationship with her … I barely know his family.
What do you want for your daughters, going forward? Increasing disparity and resentment or a harmonious childhood and teen years where they both have a more or less equal shot at life? Toys and books can be shared - but what happens when it comes to sports opportunities, drama classes, music lessons, private tutors for subjects they need help in? What about the chances of attending particular schools? With only three years between them they’ll soon notice disparities in ability to socialise, to buy clothes, to travel with friends, get haircuts, visit distant universities or take work experience at the other end of the country … You cannot have a situation where the younger is given free driving lessons and a car after watching the elder battle through part time jobs to provide for herself …
I don’t know the answer - but you’re going to need to search for one.
Maybe - even though you don’t know your younger daughter’s family - you could arrange a discussion with them where you set out the difficulty. Get them to see (to imagine) how the growing gap in life opportunities is likely to poison your family life and breed lifelong resentment. It sounds as if they love the daughter who is related to them - but are perhaps just a bit thoughtless. Clearly they have no reason to feel any responsibility towards your elder child - but if you set out the difficulty and show them how worried you are about the effect on both girls - I wonder if they might not begin to understand?
With the optimal result being that they start to offer more considered support.
Perhaps taking both children out for day trips or weekends occasionally?
Buying larger things that can be shared. Family National Trust membership, the latest technology, tickets to exhibitions or theme parks.
Paying for or contributing to the cost of holiday cottages or flights and accommodation for the three of you, rather than just taking one child away.
Transferring money to your account (ostensibly for the younger girl) which you’ll all know can be used for the benefit of both.
I don’t say this sort of conversation would be easy. But it’s necessary. Either that or you cut off all contact. That would even things out, too.