Here one has to first of all tell us what is the character of the said property and is it ‘self acquired’ or 'ancestral' or its generic nature as well as if your grandpa died 'interstate'.
Assuming the property is 'ancestral' then in that case your step mother i.e. his illegal second wife would not have any claim on the said property even if she would have been alive which she is not as per facts placed here.
But, all her children (i.e. step your aunt/uncles) have equal share in said property just like you.
Thus, the property left behind interstate by your grand father will get divided in the following manner.
1/3rd to your deceased father (assuming your father is no more as no whisper of him I found in your brief)
1/3rd to your mother (assuming she is still alive)
1/3rd to you.
Here the 1/3rd share falling to your deceased father has to be divided equally between: your mother, you, your step aunt or uncles.
Assuming the property is not ancestral then in that case the property will have to be shared equally between your mother, you, your three step aunts/uncles.