She brought personalised Easter eggs for cousins’ kids and my youngest. I immediately went over and said my eldest would be jealous in a jokey way but she had no idea that my eldest even existed!
You immediately went over, and started talking about your eldest being jealous - in a jokey (to you) way? Were there really no other options you could see as to how to approach this?
Sister-in-law had to have her own niece once a year or so ago and husband came back and asked me if I would let youngest go out with them but I said no as I think my eldest should have been included.
Why? The niece and your youngest are the same age. Your eldest is 5 yrs older. It's actually perfectly normal for playdates to happen by age. Do you insist your eldest goes with her younger sister on playdates with school friends? No ofc you don't.
Her daughter, my child’s aunt is always present and has kids now. I am resentful of her not pursuing a relationship.
Why do you think she doesn't pursue a relationship with you?
MiL and FiL are always nice to eldest but save for youngest, when I found this out I asked how much as I wanted to match it myself for eldest but they refused to tell me.
This just isn't normal behaviour and you'd have got their backs up. Absolutely none of your business frankly. Her father's parents can save what they like for her. You can save what you like for your daughter(s).
He says MiL has intervened on behalf of my eldest but has taken opportunity to moan that she hasn’t seen youngest on her own without eldest in two years and now wants a set time that she can see her. I am so upset that the mere presence of my eldest is somehow getting in the way of a grandparent’s relationship with my youngest.
If you've always insisted they are together on these visits then yes, she's right and you are being ridiculous. As a grandchild I wasn't always with my grandparents, with all my siblings, at the same time even if we were all visiting together. Relationships have different dynamics and it is perfectly normal to enjoy one on one time with individual relatives.
She will see her sister being called for a photograph with her great-grandmother along with every other child in the family apart from her. The concern that they have is my reaction to this photo and that it’s executed properly not that my child will be upset and why the fuck can’t she be in it anyway?
All of the above plus the below, and you've threatened to divorce her cousin. Why would she want your child in a family photo? Or you could explain to your child that she has granny x and y, youngest has granny y and z and this is a photo with granny z's grandchildren. I'd have thought this would be an easier conversation to have than the one explaining to her why you and her adored step dad will be divorcing.
I wanted us all to decline NOT ask for her to be given an invitation.
Controlling much? It's your DHs brother's wedding. There was a solution to be had and it was. You were all invited in the end. It's not all about you or is it....
I was finding the wedding invitation business really quite baffling. She's 10, very young, she's part of DHs family, what's the big deal? Didn't buy the bit about the phalanx of adult steps on SIL's sister's side having to come too and it being a fairness/numbers thing. Made no sense so there was a real reason, just one no one could feasibly share with you. But I'm getting the impression that you are alienating your DH's family by your desperation to have your eldest included wherever you perceive her as being left out. You want her to be treated exactly the same as your youngest. Full siblings are not treated exactly the same by their family because they're all different.
You are doing absolutely nothing for your eldest's self esteem by the horrendous way you go about tally keeping on your in laws' interactions with your daughters, ensuring that everyone treats your daughters exactly the same and am now wondering if this isn't what led to your eldest's exclusion from the wedding. BIL knew that if she were there you'd be seeing any and every instance of differentiation between the 2 as a slight to your eldest and going "immediately over" to have a word about it, probably with a tearful eldest in tow. All especially likely given the youngest would be a flower girl with everyone cooing over her and making a fuss. If the eldest wasn't there then that disruption wouldn't happen so he (erroneously) thought that would be the solution. If this is how you've acted since marrying your DH I can see that they may find you and your eldest's presence at family gatherings as pleasant as wearing underwear made of 40 grit sandpaper. You need to fix your approach to this, for the sake of your eldest's self esteem and her sibling relationship. Even if, as you believe, you have shielded her from what's going on behind the scenes, she may soon hear things, unfiltered, from kids who've heard their parents discussing you and your eldest and this WILL be very hurtful to her.
Secondly, if you do divorce, ofc your eldest will never see these people again nor have to suffer real or imaginary slights but she will still see your youngest spending half her week with her former step father, she'll hear about all the great things they do together, she'll see the gifts. She'll see her enjoying the life she once had. In time as a later teen, she'll see her go to private school and then she'll see her inherit a lot of money. She'll see her benefit from all that privilege. And there she is with a deadbeat dad she doesn't see and a step dad she loved but who can't be in her life anymore. The fact that she isn't actually his family as he's not adopted her - and maybe can't if her bio dad won't consent - spelled out loud and clear.
You'll have created a new wife vacancy too and your DH may remarry and have more kids (probably with someone childfree). So your youngest gets her own step family and possibly a taste of what your eldest has experienced. Her money, privilege and private education will not compensate for that. Is all that what you really want?
Can you honestly not see how very fortunate you are with what you and your daughters have right now even if it's not 100% perfect? You have said yourself that you wouldn't know your eldest was not your DHs daughter by the way he treats her. That is huge: she is benefitting from all that love and security and you want to swap it out for the above? I do think you have a tricky situation to deal with re the private school education for your youngest and not your eldest (but that is not without a solution, it may just not be the one you want, whereby your DH pays for both) and inheritances coming sooner rather than later but I don't think divorce will solve much without creating a different but equally complex and intractable set of circumstances elsewhere, not least of which risks being a permanent schism between the 2 siblings because they will realise that the dynamics surrounding each of their mere existences has led to them losing the loving and stable family they once shared.
You would be doing both your daughters a huge favour if you helped them to understand that life can be very unfair at times, people can be mean, thoughtless and hurtful whether intentionally or through ignorance, help them understand what really matters and what doesn't and equip them with the tools they need to develop a very strong sense of self worth and self belief so they can navigate this life successfully and not be left feeling crushed by its vagaries.