Based upon what you've said, it's probably a good idea to remove your niece as beneficiary of your 401(k) plan and your life insurance policies as soon as possible. You really don't want to procrastinate here.
If you want some or all of your assets to go to your other niece and then to her children, consider making that niece the beneficiary of your 401(k) plan. In that way, your other niece could defer the taxation on your 401(k) by rolling it over to her own IRA. You indicated that you wanted to help that niece with her retirement and that's exactly what the money would be best suited for. If you did that, then your niece would probably name her children as the beneficiaries of any monies remaining in her rollover IRA, which accomplishes one of your objectives as well.
If you did that, then you could leave certain assets outright to your niece (or other beneficiaries) from your living trust immediately upon your passing and defer certain other assets for your niece's children until whenever you specified under the trust instrument. While those assets remained in trust for your niece's children, your niece could serve as your successor trustee. Who better to look after the financial best interests of her children? If that is not a workable solution, then I believe your interests would be best served by choosing an attorney who can prepare your estate plan and give you the assurance you need that your wishes will be carried out after you're gone.
One thought about naming someone as your agent under a power of attorney and a health care directive. If you don't have someone that immediately comes to mind as someone you'd want to serve in either of those capacities, then consider not naming anyone to those positions. Instead, let the probate court appoint a conservator for you. A court appointed conservator will be subject to court scrutiny and court approval for major decisions. An agent under a power of attorney is able to do everything without any supervision by a court or anyone else. Moreover, you can avoid most need for a power of attorney by making sure that your assets are held by your living trust. If you go that route, then you should make a document known as a "Pre-Designation of Conservator." This is a written declaration that simply says "If I'm not capable of making decisions for myself and a conservator of my person and/or estate is to be appointed for me by the court, then I request that ____________ be appointed as my conservator." Courts will generally follow your recommendation unless there is a good reason not to. Then give this declaration to your attorney and anyone else you believe should have it so that people are aware of it if you are ever in that position.
As to an advance directive, you don't have to appoint an agent to make health care decisions for you. You should, though, have a living will and a writing that explains what medical teatment you do or do not want if the situation arises. These documents you should leave with your primary physician to be placed in your medical records so that they will be available if needed.