I've never been in your situation, but I agree with you about not wanting to have "those" conversations when grandma is still alive and doing well. I find it very crass to talk about inheritance when the owner of the property is still doing well. My sister and BIL are like that. I bought my home from my mother; it was a second home that she and my dad purchased for my younger sister. Unfortunately, things didn't work out and I ended up buying the house from my mom. Mom bought the house before the housing prices sky-rocketed and I bought it from her at the height of the market. She sold it to me for just over what she paid for it, so she did make some money on it, but not much. Now my sister and BIL believe that any inheritance I get from her should be reduced by the difference between what the house was valued at at the top of the market and what I paid for it. They believe that that's my inheritance.
I've told them I really don't care about an inheritance. I am doing just fine and they can fight my younger sister for everything until monkey's fly because I'm simply not interested!
As for your situation, it really depends on how grandma changed her will. If she left her estate to all of her surviving children equally then you get nothing unless she, like she said, specifically left you a specific dollar amount. If not, you would not inherit your father's share because the will reads "surviving children." If she just left it to her children to divide equally, then you and your siblings are, as a matter of law, entitled to your dad's share and there is nothing any of your aunts can do about it.
Bottom line, you need to see the Will to see the exact language in order to determine whether you're entitled to anything. If you are, go after it! Grandma wanted you to have it and you should have it.