To be upset over this

What you did in the past for them has nothing to do with how they behave with their son. That’s not your business.
You didn’t mind doing things for them with the odd bag of chips showing appreciation, because you saw your arse over not getting that flat for nothing 🤷🏻‍♀️

By all means don’t help them with things if you don’t want to do them. Regardless of his behaviour, he is their son. They will love him in an entirely different way to the way they feel about you and, having had only sporadic contact with him for so long, will be desperately trying to keep his interest now that he’s back in touch with them. It’s a sad situation but my first thought wouldn’t be to compare them trying to have a relationship with their son to what they do for you.

What have you needed them to help you with? Have you only helped them with the expectation of them helping you in return, or helping them because they are PIL you like and have a good relationship with?
Bit odd to take umbrage at them helping their son. He’s their son no matter what, regardless of where he’s lived or him not helping them up until now.

I know how you feel, but you can't compare your relationship with your in laws with the one they have with the prodigal Bil.
Don't do things for them if you don't want to, or if they don't appreciate it though.

I can understand your hurt, but please understand them. He is their son and they love him. Do not think about your hubby’s inheritance, otherwise it could make you bitter.

Why is your husband doing so little of what they want help with, that you’re picking up so much slack that the BIL’s behaviour has broken the camel’s back?

Blood is thicker than water, OP.

My BIL lives across the other side of the world. He pops over once every ten years or so and is lorded and feted by my MIL. Meanwhile DH is a dutiful son and gets sniped at.

C'est la vie.

Mums and sons, don’t try to fathom it, op. My parents are trying to gift my brother a place in Spain and don’t think I know.

you can’t change them so stop giving it any time, back off on the help

You are not their child. It really is that simple. They will never love you the way they love their children, and I can imagine that they are desperate to rebuild a relationship with him, no matter how poorly he treats them.

Pull back at stop helping them if you believe you are being taken advantage of, (which you are). Your husband can deal with them.

How are you helping him when you’ve never met him?

They're probably making up for lost time. And let's be honest, as parents we'll always help our kids no matter what.

Are you maybe jealous? This is their son. Of course they will help him. It isn’t a competition between you op. And as much as you maybe like a daughter, you are not their daughter. This doesn’t mean they don’t like or love you, but you need to be able to respect the fact he’s their son and not try to compete or compare.

Now resentment has set in, you have to pull back from them OP. Focus on your own parents and leave them to it.

Yes, I think you are being unreasonable. He’s their son, back in their lives after a long estrangement. Surely you can understand their motivation? Can’t you find it in yourself to feel sorry for them rather than angry with them?

Re the buy-to-let. Sounds like they have more than one so I don’t think your H’s “inheritance” as you put it, is at risk, though I really don’t like people thinking of their parents’ assets as their “inheritance” in that way.

If you feel like you aren’t appreciated for all the things you do for them, of course it’s OK to do less, though I’d reduce gently and gradually rather than stop immediately because you’re miffed about the BIL stuff.

“They always lead me to believe I was a daughter to them, it never felt like it to be honest.”

This doesn’t make any sense to me. How can you lead someone to believe it? And what does “like a daughter” mean ?
To them, treating you like family might simply mean they’re happy for you to know their business, helping with paperwork and medical appointments. It sounds like they just come out with meaningless niceties about it. The reality is - in laws almost never see anyone but their children, as their children.

Has something changed recently? Prodigal son has been back 2 years already. What drove you to post today?

Making a fuss of birthdays etc has got nothing do with being busy running a business. You’ve willingly - by the sounds of it - covered your husband’s slack picking up the wifework. You’re right it should be their children not you, but that includes him. Fair enough if you had more spare time (funny how the men are always too busy to make a fuss of mum’s birthday though…) for something like going through some paperwork. But the fuss of a birthday, showing you care about someone? That should be both sons, not you - sounds like you’re finally fed up of picking up the slack for both of them!

“Like a poster quite rightfully mentioned they claim to see me as a daughter.
But im not their daughter.”

I don’t know why you read so much into that. It’s just meaningless niceties. People come out with shite like that all the time! “Oh she’s like the daughter I never had” is just an easy compliment. It’s genuinely meant as a compliment - but they don’t actually mean it. It’s easy to say things like that… but say you cheated on their son. They wouldn’t be torn because they still love you despite your mistake, would they? They’re not thinking about what it means to truly feel someone is your child.

Just do whatever you have time for and feels right - and let them do the rest.

And don’t get stuck running their BTLs without a wage!

They're trying to build bridges with him. Step back and give them time. Hopefully your BIL will realise that he needs to step up and help them in return.