Disclaimer: The materials provided below are informational and should not be relied upon as legal advice.
I am sorry for your loss. During her lifetime, your mother could choose how she wanted to dispose of her assets, including any life insurance policies she purchased. If you believe your sister unduly influenced your mother to name your sister as the life insurance policy beneficiary, or other foul play was involved, you cannot challenge this. If your sister was personally the beneficiary of the policy, she can choose to spend the proceeds as she wishes. Be sure to consult your own attorney to protect your legal rights.
I'm very sorry for your loss. As Attorney Marshal suggests, the beneficiary designation will be difficult to challenge unless you have evidence that your sister forced her to make the beneficiary designation under some sort of duress, or evidence that your mother lacked the capacity to contract with the life insurance company at the time she designated your sister as beneficiary (for example, if she had dementia).
The proceeds of a life insurance policy pass to the beneficiary by operation of contract (rather than, say, according to your mother's will, and regardless of any oral statements your mother might have made expressing a hope that your sister share the money). The beneficiary can spend the money however she likes.
If you believe that your mother was not of sound mind, or that she was under some sort of pressure or duress when she designated your sister as beneficiary, and you are thinking about challenging the beneficiary designation, talk to an attorney as soon as possible; there are time limits at play here.
The proceeds of life insurance are awarded based upon the beneficiary designation in place at the time the insured passes, i.e., such proceeds are a non-probate asset. But there are exceptions, including but not limited to, where a designation is procured through, fraud, duress, undue influence, lack of legal capacity etc. You do not suggest that such factors exist. If you believe there may be a legal basis for attacking the validity of the designation, you should contact legal counsel in the applicable jurisdiction, which I presume is California, at once. Best of luck to you.
The foregoing is a general answer based upon limited information, should not be construed as legal advice , and does not create an attorney-client relationship. It is the opinion of the writer alone. The author is licensed in Indiana and Ohio attorney only.