In California, married couples have what's known as a spousal fiduciary duty to one another under the state's Family Code. According to the code, the marital relationship "imposes the duty of the highest good faith and fair dealing on each spouse, and neither shall take unfair advantage of the other."
This means that if one spouse is incapacitated, the other can manage any assets that are deemed community property. However, in order to manage his or her spouse's separate property, the spouse must have power of attorney or trustee powers.
This is yet another reason why estate planning is essential. If you want your spouse to be able to manage your separate property should you become unable to do so, you need to designate that. If you don't, your spouse may need to file a petition with the court to manage non-marital assets belonging you.
It's best to make these arrangements in your estate plans while you are both physically and mentally healthy. Otherwise, it's possible for other family members to make a case that you weren't mentally capable of making the decision to let your spouse manage your property or that your spouse used undue influence on you.
When you and your California estate planning attorney are developing your estate plan, it's generally best to inform your children and other family members about the terms of the plan. This is particularly important if you are on a second or subsequent marriage and have adult children.
While this is often less than pleasant, by informing everyone of your wishes, you can help prevent family squabbles after you are gone. Your attorney can advise you regarding how best to do this and can be on hand to help answer questions and deal with any issues that family members may have.
Source: Record-Bee, "The spousal fiduciary duty," Dennis Fordham, accessed May 31, 2017