It has a second definition, this is a "Grand" MILF, similar to "Grand Canyon". A GILF is above a MILF in the hierarchy, if she is still beautiful by traditional standards. She is the high priestess in a sea of MILFs.
If you're a milf and your kid has a kid, you are now a Grand MILF, regardless of daughter's age.
I hooked up with my friend's Grand MILF once.