Frankly, your post doesn't make too much sense - you have a lot of conflicting terms in there. Life insurance policies don't have trustees. She can't close a joint account without the co-owner's permission. What do you mean by "irrevocable life insurance policy"? Who is covered by the policy and who owns it?
Perhaps it is an irrevocable trust set up to own/fund the life insurance policy on the parents. The bank account is for "gifts" that pay the life insurance policy, with the trustee(s) having check writing ability.
However, the trustees, for tax purposes, have to notify the beneficiaries with a Crummey letter every time money is "gifted" to the account, and their rights to exercise withdrawal in certain time frame (30 days). Perhaps OP's siblings seek to get OP to sign a blank piece of paper and then print a Demand Notice on it, waiving OP's right to the OP's share of the "gifted" money, so that the premium can be paid. Who knows. I do agree that signing blank pieces of paper is not a good idea. I also question who "they" is.
I think OP should reread the language of the trust document, and perhaps meet with the law office that wrote up the initial document, because the description sounds very different from what I am accustomed to. (I am the trustee of an Irrevocable Life Insurance Trust (ILIT). I see no way, as a trustee, that I can alter the beneficiaries - nor would I wish to.)