There are different power of attorney forms for different purposes. If you are talking about a "power of attorney for health care decisions" (also called an "advance heathcare directive"), California law specifically permits it to be witnessed by 2 "disinterested" witnesses instead of being notarized. But one of the witnesses needs to sign a special clause that the form should contain - the clause indicates that you knew what you were signing, etc.
If you're talking about a DPA for Finances, I'd strongly suggest that you have it notarized.
The problem with using forms off the internet is that they are not necessarily geared to the laws of the state in which you need to use them - and most of the time, you don't have the time (or the ability) to muck around with "getting them right" when you really DO need them. I find that using "generic" internet forms for documents that are needed in dire circumstances is really "penny wise and pound foolish". Why not spend a few extra dollars and get documents that you can be sure will work when you need them to?
This information is not intended to substitute for professional legal advice and does not create an attorney-client relationship. You should accept legal advice only from a licensed legal professional with whom you have an attorney-client relationship.
Typically the notary can be substituted for the witness signatures. The safest practice would to have both; that way your covered. It is not a good practice to have family members as witnesses for reasons to numerous and complicated to discuss in this forum. Have uninterested people witness the signature.
Disclaimer: I am not offering legal advice, assume I do not know the law in your state and that I am just making suggestions for starting points for when you do speak with an attorney. Do NOT rely on anything I write and contact a lawyer in your area immediately after reading my posting.