Originally Posted by Brat
Estate taxes are determined by the value of the estate at the time of death. There are some techniques lawyers use for very large estates that avoid taxes but they must be established prior to death.
A revocable trust will enable your mother to describe exactly how she wants the assets in the trust distributed. She should also have a will that scoops up any assets that aren't in the trust at the time of her death. Frankly I recommend that you have a lawyer familiar with estates prepare the revocable trust and her will. If you draw up the trust other family members may claim that you had inappropriate influence. Your gift to your mother could be finding lawyers who would have her desires primary and paying for her estate planning.
Keep in mind the fact that it isn't enough to establish a trust, you need to put the assets in the name of the trust. This is a step many overlook.
X2, your mother has revocable trust, and it distributes the proceeds. The distribution is where you can have one of the recipients be an irrevocable trust for your sibling. When your mother dies, the revocable trust becomes irrevocable with someone as trustee to handle the distribution of assets, per the trust's instructions. You need to talk with estate attorney for your mother's state, and they can write it up to do what you need.