Is it legal for a successor trustee not to be allowed during the will versus trust discussion?

Its not illegal.. that is most likely the standard practice of the attorney.. I might also institute this practice with an estate planning client if the person being excluded gets special benefits (e.g. more than your statutory equal share) from her estate because is probably a good idea that you be excluded from the discussions so that attorney can be sure you do not have some sort of undue influence over her actions so that the will and trust can not be challenged after she passes... Ask to talk to the attorney separately so as successor trustee he can explain the estate plan's provisions to you.

Yes, it is your mother's will.

That is partially correct. Some parents do have their adult children meet with them and their attorneys but it is not required. It is the client's decision whether to have a third party in a meeting with an attorney. If your mother brings you into the meeting she has no confidentiality concerning whatever is discussed while you are present. Her attorney may have advised her, for whatever reason, that she should not give up that confidentiality. Whether she executes a will or a trust is really her decision to make. As a successor trustee you don't actually have a role until the trustee(s) before you either die or resign. I assume your mother named herself as initial trustee. You don't mention if there is someone else in line before you as successor. But regardless, a successor has to wait for his/her turn. That said, as a practical matter, I advise clients to talk to those who they name to such roles to be sure they are willing to accept the role. And clients are free to show their will or trust to whoever they want. But that doesn't mean they have to meet with me.