The instructions below will get you started with the procedure to receive ITINs for your siblings.
How to apply ITIN for child while filing tax return? (or dependent)
The instructions for applying for an ITIN are included here for you. The tax return should be prepared using 999-99-9999 for the child without an ITIN. When the return is printed you should remove that number and leave it blank. The Form W-7 will be attached to the tax return and the IRS will issue the ITIN. Expect a long period, but as long as you apply for it they will issue it and process the tax return.