Stepchild wants to see finances and money spent on his stepsiblings.

You really are totally mistaken here. Please go back and read the OP's posts.

"...would you not feel a bit protective about your DH spending it on dc wholly unrelated to said grandfather?"

Surely you mean ex-DH?

The father is not spending any of the child's inheritance at all.

The grandfather very clearly gave the benefit of the capital to the grandchildren (which the children cannot touch).

He also very clearly gave the benefit of the income to the children. What the children choose to spend that income on is purely down to them.

You shouldn't be in the middle of this; your husband should be handling it and shielding you and your children from any fallout.

Yup. If my dc and I were benefitting from funds that originated from an unrelated Grandfather, I would be pulling my head in and - and certainly not berating his real GC for asking about the details.

Well to be fair, if your dc had a wealthy grandfather who had left money down through his descendants, would you not feel a bit protective about your DH spending it on dc wholly unrelated to said grandfather?

There is only one answer. Not his money not his business. Your dh needs to nip this in the bud. Entitled little so and so. I would wonder where this is coming from. Not a usual 18 year old question.

You are correct about the money from the grandfather that is left to the grandchildren.

However, the OP (in a later post) has said that DSS is aware of how the not-a-trust operates and he is more concerned about any expenditure by his father now that will mean that the father's estate is less worth than it would otherwise be and so he and his sibling would inherit less from his father when he eventually passes away.

That really does sound like the ex-wife (his mother) is putting these thoughts into his head.

Because based on a previous thread by the OP the stepchildren’s mother feels that any money spent by the DH on OP’s children is taking money away from her children.

So it seems like any treats the OP’s children are getting are being monitored to check who is paying.

I would guess that the DSS would be worried that his father had paid.

What nonsense is this! The actual grandchildren are not going without at all, where on earth did you make that up from?

You do not know that is true. It is actually very unlikely that the original trust from FIL would be set up this way. It is more likely that all FIL’s assets have to pass to his biological descendants.

I can see them too. Dad is spending on his new wife’s kids. Of course he isn’t happy about it. There is probably some jealousy too.

but why is the husband paying for his stepchild’s school trip for example? Shouldn’t his dad contribute?

It's up to your husband to say no and to tell his child - both children, if necessary - never to make this kind of demand again. If my 18yo did this, I'd be shocked but I'd hope it was a freak moment of immaturity and self-centredness, perhaps mixed with some misguided protective feelings toward the younger sibling. If he genuinely does not understand why it's outrageous and offensive, his dad should explain. I would hope it wouldn't automatically mean the end of the relationship, though - especially not with the younger child who hasn't done anything wrong.

You shouldn't be in the middle of this; your husband should be handling it and shielding you and your children from any fallout.

There’s a PP where OP clarified that there were no spending limitations on dividend received.

It appears that the capital
is in trust and your husband and his siblings get the interest from that capital (however it is invested). Once DH receives that income it is entirely his to do with as he sees fit. Don’t put yourself down by suggesting he is subsidising you. You are a partnership and can make joint decisions.
Your SS is being an entitled brat expecting anything spent on you or your children to be paid back but there is one aspect of the situation that he might be right about. If the terms of the original will suggest that there should be something for the grandchildren to inherit then it is not unreasonable for the grandchildren to be concerned about the way the grandfather’s money is invested and managed. He should be made aware of who the trustees are, what assets form part of the trust and what investment criteria are being used. It sounds as though he is not yet mature enough to deal with this information sensibly so if I were your DH I’d be telling him that he will discuss it with him when he is older.

This

I think the OP says the SIL lives frugally because the 'trust ' money is her only income.

OP's husband, on the other hand, earns his own income as well as receiving the 'trust' money.

Yes exactly this your DH isn’t dead yet!!

tell him to stop being a greedy sod and do one!

Where did you read that OPs SS is going without?

There is no mention in the OP, but then the OP wasn't even clear if it was actually a trust.

You would have to see the Trust Deed to know there are no limitations.

My aunty came over and took my daughter to a show. When she was telling her step-sibling about it the immediate question was who had paid

I'm confused - this is your aunty and your daughter, so what does it have to do with his bitterness about his father's finances?

I wonder if the OP regrets this thread now? Dozens of strangers calling her stepson a greedy fucker, spoilt little shit etc when he's probably just a confused, insecure teenager who's been let down by the adults in his life

if he continues down this arrogant path of thinking he’s entitled to wealth because of the sperm-based component of his DNA he may find out he gets none at all!

But hang on a minute: as far as funding from the trust is concerned, op's DH is also only entitled "because of the sperm-based component of his DNA" (what a vulgar description) as well - and the OP not even that, her dc still less again.

So what does that make all of them if the DSS is arrogant and entitled?

But the father does earn his own income.

The money from the 'not-a-trust' is in addition to this.

Your DH just needs to say no you can’t have access to my finances but you’ve got nothing to worry about because when I die you’ll be fine financially

I haven’t read all the posts OP but I have read yours. Your step children have no business seeing your DH’s finances. The only bit they need to be concerned with is the capital settled by their grandfather, and even then, only once it gets to them.

Since you say your DH cannot deplete the capital, there’s no issue. They will in due course receive the entirety of their grandfather’s settlement. Your DH needs to explain this calmly and firmly. None of the money coming to them has been spent because the “not quite trust” forbids it. They either believe him or they don’t but they will see for themselves once the trust reaches them.

Even if his father is prioritising his step children over his own children, he can be pissed off. It doesn’t give him the right to have access to his father’s financial records or try to “backdate money owed” from his stepmother!