I’m torn here.
You weren’t married and are nothing to do with that trust (ie not a beneficiary) so why do you think you have any right to sight of the trust deeds ? It’s nothing to do with you.
Like it or not, your children are his too and any private arrangements he puts in place to provide income for either himself or the children are not your business.
Your ex is paying maintenance for his children. You might not think it’s enough (and morally it might not be based on his total income), but there are rules set up regarding the level of maintenance payable - go through CMS if you think your children are entitled to more (although I suspect you’d not be).
In terms of inheritance, it’s up to your ex how he deals with this. It may be his family’s money and they don’t want you accessing/controlling it (relations there sound acrimonious at best) or it might be him that wants you no where near it ! It’s up to him how to safeguard his assets for when he dies. He may prefer his brother administers the trust and trusts that he’ll ensure the children are catered for. He might not want to leave any to his kids ( I’ve no idea if they get on, their ages etc... I personally couldn’t disinherit my children but some famous/rich folk do - Simon cowell is an example - thinking it benefits the child to have to make their own way and not rely on others).
Doesn’t sound like you two trust each other at all. You sound very bitter about him / his family. Not sure of the back story there or if you’re just mad you had children without the protection of being married. Either way, I’d stop focusing on him.
In short, you are not entitled to anything here. Follow the rules to get what your girls are entitled to in terms of maintenance if you think they are being short changed. With inheritance, it’s never certain as he could spend it all before he dies. Just forget it.