Yes Siblings can use the stem cells from the cord blood but if the patient has any genetic disorders they cannot use their own cord blood and would have to use some other cord blood which could be their siblings cord blood.
If your family has a history of cancer or Leukemia you should always if you can bank your babies cord blood to have the cord blood on hand.
Cord blood can be banked for many years and can last for 25 years or more.