My 86-year-old widowed mother has no trust or will. How necessary is it at this point?

A trust or will is not necessary if all property and assets are held in accounts or other form that has a beneficiary designation. If she owns real estate, some states even permit the use of a beneficiary deed, but this is a minority of states. Her personal property is technically a probate asset and would pass by will or your mother's state's intestacy laws, but if the personal property is modest in value and you don't expect any problems dividing it up, then that should not cause any issues.

Often a simple will is helpful in even such cases as yours if your mother wants to put one child in charge of dividing things up, or if she wants to leave certain items to one child and other items to another, or if she wants to divide monetary assets (stocks, mutual funds, bank accounts, etc.) unequally. However, if every single asset is covered by a beneficiary designation and she is happy with that, then a will may not be required.

In my (limited) understanding, "trusts" protect a person's assets. In essence, the assets under a trust no longer belong to anyone in particular, they belong to the "trust." A trustee is then appointed to help with investing and managing any funds in the trust. Some people create a trust to protect assets if they should ever have to access Medicaid funds. There is currently a look back period of five years.
If your Mom has few assets, has already designated beneficiaries for each, and would never need Medicaid assistance, a trust may not be necessary.

I would contact someone who specializes in Elder Law... I had an issue with my folks(they are in their 80's) about a will, medical power of attorney etc... theirs was very outdated... It gave me custody of my younger sister, we are both in our 50's! I told them I would not argue with anyone for "stuff"... that seemed to motivate them...

If your mother dies while on Medicaid, depending on your state, most probably, her assets will need to be sent to the state to reimburse the state for Medicaid. This may be true even if she does not die in a nursing home, just because she is 55 or over. If there is a will, I don't think it matters who is to get what. Medicaid is required to be repaid first. Even if there is no Probate, Medicaid is required to get assets. If there is no Probate, they will contact the family.

I have no will in place...how do I begin...an atty in San Francisco?