Keep in mind that I make a living as a librarian and as such am paid big bucks to research fun stuff. I am a trained professional. Do NOT try this at home.
First, why on earth would anyone think of Kevin Mmahat (which is pronounced Momma-Hat) unless you were making a list of over-hyped New York prospects (and there are so many better options than Mr. Mmahat)?
Once I saw the next two cards, it seemed obvious to me.
Momma, Daddy, Granny. We've got a family thing going on here.
Then the next three get tricky:
My first thought was that there was some sort of Jewish connection. Portman is Jewish. Stein is a common Jewish surname. And people thought Greztky was Jewish. But he's not. Couldn't come up with anything there.
I thought for a while that the fact that the Gretzky card was a "Trilogy" card might be something since we had a trio of family members. That seemed redundant, though.
Were the trio the family? Portman is Momma, Gretzky Daddy and Frankie is Granny? Doesn't make sense.
Gretzky and Portman both state they are limited edition cards. Something there? Probably not.
Portman is hot, Frankie is not. Gretzky is not. No relationship again.
The Gretzky card is numbered 15/30 which is 1/2. Frankenstein is half man, half machine. No wait, that's a cyborg. Scratch that.
Sticking with the numbers, maybe the 15/30 is really 1530, the year of the Augsburg Confession and also the year Julio Franco was born. Julio Franco would be 476 in 2006, the year on Portman's card but then once again there was no connection to the Frankenstein card unless Franco went to high school with Mary Shelley.
By this time I was getting frustrated. So I did what anyone would do. I Googled Momma, Daddy, Granny. First result had the number 99 (Gretzky was the anti-Lemieux) and it all fell into place.
Interestingly, if I hadn't typed Momma as Momma and had typed Mama, I might not have figured it out and then we would never have known the answer. I don't think anyone else stopped looking at the Portman card long enough to try and figure out the rebus.