This sounds more workable but the OP implied the beneficiaries will not be given money they can redistribute, but that those items/fees/costs will be paid for by a trust.
I’m not sure that there’s a huge benefit of being a trustee, rather it’s a necessary management system.
The question is how will step children become eligible and once eligible, can they become ineligible?
I guess there could be an option for a relative who becomes a step parent to have named children added to the “family”.
There would have to be clear eligibility criteria — children under a certain age at the point of marriage or children moving into the family home — rather than living with their parent but part time with children? Longevity of relationship or something?
The issue with situations involving future, theoretical, non-adopted/non-biological children is that there is no reliability on their involvement with the family. The partner who brings them to the family is an entirely unknown and possibly uncommitted factor.
Reasonably, you have to cater for worst case scenarios. Imagine the worst possible partner for one of your children: dishonest, disloyal, manipulative, gaslighting, and then consider how you want to cater for his/her children.
Surely the legal advisors can create clear criteria for eligibility for the trust’s benefits, even if it’s a list with applicability being at the discretion of the trustees excluding the stepparent.
Complex.