Leaving the cost of the valuations aside, you are going to need the parents consent and a parental indemnity from the parents if the beneficiaries are going to agree to this because the parents are going to be acting on behalf of the minor children.
It seems that the situation is that whereas the children could get something now, if it carries on as it is, when you and your sister eventually die, there is going to be nothing left.
What you need to do of course is convince the parents that it's in the best interest of their children's finances to take a chunk of money now rather than wait until you your sister died.
They are going to need to take legal advice on this and it's only fair that the trust fund would pay for that legal advice.
The trust is going to need to be valued and an estimate of the future value to the children put on the trust assets for them to decide whether they would be better off waiting or not.
It is the valuing of the trust now and the valuing of the trust using projections in the future which is probably going to be the difficult part although any competent actuary would be able to do without although it's not going to be cheap and I strongly suspect the fee for that alone would be a couple of thousand pounds.
In the other beneficiaries of the remainder are minor children, you have to convince the parents to wind the trust are now rather than wait. If they are simply not interested in winding it up now and they would rather wait for their children to get whatever's coming, but then that's the end of the matter.