This will depend on what your grandmother's will says. You can't give something under a will to a deceased person, so there isn't a 'portion' of the estate which is his. Here's two common examples of what wills can say that may give you some guidance on what could happen (expressed very simply, so forgive the crudity in drafting):
1. Will says, "I give my estate to my children X, Y, and Z as shall survive me, equally as tenants in common." If X, Y, and Z outlive the testator they each get one third of the estate. If Y doesn't outlive the testator (but the others do), then X and Z get half the estate each, and "Y" gets nothing (including Y's family).
2. Will says, "I give my estate to my children X, Y, and Z as shall survive me, equally as tenants in common, provided that if any of my said children fail to survive leaving children then such children shall take the share of my estate (equally, as tenants in common) which my child would have obtained had they so survived me". Again, if X, Y, and Z outlive the testator they get one third of the estate. If Y doesn't outlive the testator but has two children, then: X gets one third, Z gets one third, and Y's children get one third split between them (so one sixth each).