Your sister in law is the Trustee of the fund. As such she MUST comply with both the trust instrument (ie the Will) and trust law. The trust instrument provides that your son must inherit the entire capital at age 21. In the meantime, the Trustee may advance the capital to him at her discretion. She must take advice on the investment, she must act prudently, she must be ready with her accounts. (You are entitled to an account of the money). Your son is also entitled to the intermediate interest on the money subject to the terms of the Will.
There shouldnt be a huge cost to the estate of setting up a trust fund since I would imagine the way to deal with a fund of this size is to put the majority into a high interest notice account and some in an instant access account. I cant say for certain but you should be able to get an IFA to advise relatively cheaply.
In terms of legal advice, I would suggest you seek the services of either a specialist solicitor or a direct access barrister. The joy of going to a specialist is that they will quickly get to the nub of the situation and thus should not cost you too much money.