In most cases, the state will not issue any tax documents such as a 1099, so the IRS will not get official notice of the income and you are on the honor system to report it. If the reason you are splitting this income with your siblings is that it originally belong to one of your parents, I would simply report as taxable income the amount of income that is your share. You would owe between 12 and 22% federal tax in 3 to 6% state tax, depending on your other income.
however, that assumes that all the money is taxable. It may not be, depending on where it ultimately came from. You would have to give us a lot more details before we could help you with that. Was this a life insurance policy? Abandoned investment account? Forgotten bank account? It really makes a difference, because the income may only be partly or nontaxable.
