1. It's not clear in your narration, whether your father had executed a WILL or had died intestate (without executing a WILL).
2. Assuming that your father died intestate, then the property would devolve equally to your mother and 7 children. In other words 1/8th share to each one of them. In case your mother is also deceased, then the 7 children will get 1/7th share.
3. In case of deceased person (in the instant case your deceased sister), her share in the property would be subdivided equally amongst her children.