The property in the name of your stepmother, though reportedly bought by your father, shall be her own,absolute and exclusive property. Upon her intestate death the property devolves on all her own legal heirs equally. Her own class I legal heirs consists of her minor son and your father. Since your father too died intestate, his 50% share share in the property shall again devolve on his own legal heirs consisting you and your siblings and the minor son of his second marriage.
Therefore your share in the property is restricted 1/5th out of your father's share from that property. The minor son of your step mother shall be entitled to i original 50% share and also 1/5th share out of your father's share in the property in addition to it.