This was an issue from seven years ago. I am surprised you held on to that for this long. No judgment or criticism intended.
I can't tell you what your mother was thinking when she said that. But if your grandmother was insensitive then that probably has something to do with the explanation of selfishness. I must ask you now if you really want to find out some answers, you must do so without judgment and with an open mind. If you don't want to know, I would let it go.