Older sibling stole money after Mom died. What's the next step to get our equal share of the insurance money?

Insurance payouts are not subject to a will. Even if a will (or oral or written promises) says everything is to be divided among 4 people, if only one person is listed as a beneficiary, only that person gets the insurance money.

Sometimes a person who divorces and remarries forgets to change the beneficiary to the new spouse. The ex gets the money in that case. Whatever is on the policy is how the policy is paid. It isn't about what is "fair" or what the deceased would have wanted. It is about what is actually written in the policy.

Who was the beneficiary named on the insurance policy? If it was payable to "older sibling" then did older sibling pay for the funeral for "brotherdied" ? Did brother died have other bills to pay? How do you know it was $50K and not a decreasing term policy? The insurance company makes a check out to the named beneficiary, period. Nor can you sue to get part of it, sorry.